I came into the Realms with 3E. Everything before that (I do own the old gray box) is just too ugly and baldy typeset for me to read. So my opinions are mostly on 3E FR.
FR has a LOT of lovely elements. This remains true in the 4E books. Many of the maps, settlements, prestige classes, and such are well tough out and interesting. The 3E maps are gorgeous. The magic is quite reasonable; it’s what you would get in a world where the main competition to high-level mages are other high-level mages. FR is a very good place to farm for elements you want to put into your own campaign world.
FR is also made-for-adventure. Because the world is so compartmentalized, you can let adventurers run amok in one area and it basically will not affect the next. Even before 4E, it had a points-of-light feel to me; FR is about small settlements in a big, hostile wilderness. Even major cities seem like this, islands in a sea of wilderness. My perception was probably shaped a lot by the Baldur's Gate series of games that took place in a FR that was mostly wilderness. The powers of the wild are real and mostly evil. Malor is not someone to be joked at. This makes it very easy to make a FR adventure; you can basically ignore the setting and just make it generic backwoods with no problem whatsoever. If you want to include your own city, it makes perfect sense that it is small and isolated and unique, because every city in the FR is.
But there is a downside to this. The world feels like a patchwork, with no cohesion. I'm a history buff, and FR, despite all its back-story, does not feel like it has a history. To me, history is about the conflicts and agreements that shape the world, about social contracts and clash of ideas. In FR, history is bad because it is monolithic (because alignment has such a strong impact), and bad in another way because it is too fractured (because there is no sense of historical progress, "ages" of history, or spread of ideas). FR history is about spurious magical events that lack root causes and just inject entropy into the setting.
For example, take Cormyr. It has a lovely back-story - which rarely interacts with the history of any other country. It has remained as it is for more than a fifteen hundred years, and if it changed during that time, that is barely perceptible. If you look at a RW model, like France, in fifteen hundred years from say year 1 to 1500, it went from a province of another realm, to a barbarian tribal area, to a part of yet another semi-barbarian empire, to a morass of feudal fiefs, to a series of warring states, to a monolithic monarchy. Yes, Cormyr has changed and has had different periods in its history, but it is still very much more the same. And so has many, many other areas of the FR. Until the Spell plague shook things up, there had basically not been more than local change for a thousand years. And now, post spell plague, most people seem to be trying to get back to what was before the plague.
That kind of conservatism doesn't seem credible to me. Then again, it might be a sensible reaction from people who live in a world that could change violently at any time - with no sense of natural progress in nature, perhaps sentients would create very, very strong traditions and cling to them. But it’s not really my kind of world.
