By the late 1990’s, TSR was in trouble. Despite having a firm basis in pen’n’paper RPGs, they fell behind Games Workshop and Wizards of the Coast in terms in volume sold. Despite not being in the same market as WotC at the time (they were focused on Magic the Gathering), and only tangentially competing with GW (whose Warhammer Fantasy RPG was available, but secondary to their miniatures), they made a push in the mid nineties to seize overall gaming supremacy, which backfired badly in 1996. Facing insolvency, TSR was forced to sell itself to WotC to remain in business.
3/3.5e D&D has its odd monicker because a few years after the initial release of 3e in 2000, in 2003, a revised edition was released. This game was the culmination of effort by game designers from TSR and from other gaming companies, and it altered the gaming landscape permanently. It did this by introducing a new, streamlined, low learning curve set of game mechanics, and then by making the game system free to use for anyone who wanted to use it through the OGL (open gaming licence). This caused an economic bubble on its own, and when that popped, near as I can tell, it caused a panic in WoTC and triggered the process that led to 4e D&D.
This edition stuck hard and fast with the three core rulebook plan from 2e AD&D, and is actually more functional and complete than the previous editions were. All that is needed for this edition is the PHB, DMG, and MM. Within them they offer a complete tool kit that is easy for both brand new and experienced groups to approach and use.
Score: 5/5 All you need are three books, and they actually have everything in them.
3/3.5e take a dramatic step away from the rules of previous editions. The d20 System simplified, streamlined, and dropped the learning curve of the game dramatically. Level caps were abolished, as were attribute caps. However, it wasn’t free of odd “balancing” attempts, based on the idea that certain racial abilities were equivalent to levels for purposes of determining what level of threat a character or party could deal with. It was clunky and didn’t function well, and frequently increased threat out of proportion to what an ability actually did. The combat also tended to get clunky at high levels. However, as a whole, it represented a dramatic improvement to flow and accessibility over previous editions.
Score: 3/5 The d20 System is a highly functional mechanics system with a shallow learning curve; but things can get cluttered, especially at higher levels, and their efforts to balance optional races was very clunky.
No edition previous or since has placed so many options in the hands of players. A revamped multiclassing system combined with new, limited scope “prestige classes” (in the DMG, and in expansion books), and a plethora of equipment, feats, magic, gods, and so on created an environment where players could achieve pretty much anything they envisioned, and DMs were often left scrambling to keep up with what was going on. Some argue it was too much agency, others that there was a lot of overlap between new classes and prestige classes (which can be accurate), but on the bulk of it players and DM’s alike have more options in this edition than in any prior or following edition.
Score: 5/5 This is the most versatile edition of the Dungeons & Dragons when it comes to player options, even if you don’t expand past the core three books.
This is a hard one. Previous editions were very generic in the core three books, but defaulted to Greyhawk without actually providing much information. 3/3.5e took a slightly different path. They presented a generic fantasy world, using the deities and their iconography from Greyhawk, but didn’t expressly make Greyhawk the default setting. The RPGA did run the Living Greyhawk Campaign, but it never made the leap to officialness.
Unlike previous editions, 3/3.5e D&D gives you more and better tools to build your own world up from. As a result, I argue that it presents a generic fantasy world of its own, one with gods, mythology, and so on; which while based on material from Greyhawk, isn’t Greyhawk in itself. Like previous editions, this world is expanded on in supplemental books, which culminated in an unnamed but vast world I wrote about previously. 
Score: 3/5 This edition puts some solid tools in your hands to kick off your own fantasy world, which while related to Greyhawk, isn’t that setting.
Low to Moderate. This edition does extremely well at having both males and females in art and isn’t afraid to dress the odd male figure up in impractical clothes either. Ethnic diversity remains low though, with only two POC (a Black and an Asian woman respectively) in the PHB, and no one visually identifiable as a POC in the DMG.
Score: 2/5 This edition really picked it up in terms of male/female diversity, but did not do so with ethnic diversity.
Decent. 3/3.5e D&D is still available for purchase, being only about nine years out of print at the time of this writing. Some stores may have a small amount of material left, so most of this will be online purchases. And of course, as per normal, there are a number of online resources of varying levels of aboveboard or below-board for electronic copies.
Score: 3/5 Things are starting to get rare and expensive, so get on it now if you want in.
To be honest, building your character for this edition takes a while, especially if you’re starting above level one. There’s a lot more detail that goes into them than in previous editions. However, the tradeoff is that the learning curve is much shallower for game mechanics, and the are a lot of options for everybody. Gameplay is smooth, and combat is sometimes crunchy if you’re high level or in a large scale combat situation. Overall the 3/3.5e D&D gameplay experience is a good one, and representative of the more user friendly system design that became more popular in the late 90’s as RPGs began targeting the non-gaming population more heavily.
3/3.5e has the largest contingent of fans still playing it for a reason. It was an excellent product that removed a lot of barriers from potential gamers and placed a tremendous amount of agency in the hands of the users. It moved away from the traditional “dungeon-delve” model and the “standard tactical party” and really laid it out there for people to get creative with. It was so popular that when WotC fired Paizo Publishing before embarking on 4e D&D, Paizo were able to create a competing fantasy RPG (Pathfinder, sometimes referred to as 3.75e) using the d20 System, that remains competitive to this day.
Personally, I enjoyed, and continue to enjoy 3.5e D&D. It’s fun and customizable, and still offers a rare embrace of high fantasy. The OGL is still active, so if you want to build your own game for fun or to make a buck, but want an established set of mechanics, you can. But be aware that it can get book heavy if you want to get the most out of it, and that its support material from Dragon and Dungeon magazines is still available.
Final Score: 21/30 This game doesn’t so much provide modern gamers with a retro experience as the previous editions did as it provides a fairly standard gaming experience. Its system is still in use and there’s still, as of this writing, a large and modern gaming contingent playing it. It’s a good experience overall, but may not be to the tastes of gamers who prefer a lighter system or less complexity in their characters, or who are looking for more strict definition around what characters do/can be.