I will say that Fallout 3 does have some "good for what it is" elements that others have pointed out - the DC ruins and other atmospheric locations, a couple of the side-quests, the visual direction and design of the wasteland, etc. I also do like some of its ideas mechanically, like repairing guns and crafting new items out of junk, but they were both poorly balanced in practice. Taken as an open-world dungeon crawler, I think it actually excelled - I still remember a few locations, like the huge sewer system with the ghoul living in the bottom of it, the Deathclaw Sanctuary, or that old military base where you find the T-51b armor. Ultimately there are definitely fun things about it and I have no problem stating I played it for around 80ish hours, like every Bethesda game, and I don't regret it - I had fun wandering around shooting and looting, especially once I had some mods installed.
Taken as an actual sequel to Fallout 1 and 2, though, the game is a pretty big disgrace for obvious reasons - awful writing, stupid quests (though at least some have some choices to make), simplified/bastardised combat and character system, etc. - and I really, really think they should have just given it a spinoff title instead to avoid associating themselves too closely with Fallout and setting false expectations. Of course, we now know that nothing is sacred when it comes to Bethesda.
I'll also say that now there is literally no reason to go back to Fallout 3 now that New Vegas is out. It has more content, better mods, better DLC, a better story, better quests, better writing, more memorable and plausible locations, and so on. I actually tried out Fallout 3 a year after playing New Vegas and it was really, really difficult to enjoy... I had fun the first time with it because I went in knowing the writing was going to be bad and just ignored it, but New Vegas sets the bar for that so much harder that in Fallout 3 it just becomes endlessly distracting.