It takes a while to get going, but it's worth it in the end.
One of the most obvious comparisons you could draw it to is Elder Scrolls IV: Oblivion. Yeah, it's pretty similar, except the whole deal with guns, but it improves on some very crucial aspects. For one, the locations in Fallout feel more important. They're not just random caves or ruins you see in Oblivion, they're makeshift shelters, derelict power plants, towns cobbled together with sheets of scrap metal. They feel important and unique whereas the locations in Oblivion felt like the same ruin over and over again. I can easily say that half of my 45 hours spent in the game was just wandering around, and there are still huge tracts of the map that I haven't yet explored.
Skill leveling has also changed for the better. You no longer need to repeat actions to level something up (...I can't be the only person who've spent an hour sneaking behind some shop keeper to level up sneaking). Over all, they've made some pretty important improvements from Oblivion, and it shows.
So. Downfalls? Most notably, the gameplay. VATS works, gunplay outside of it doesn't. And it really doesn't. Yeah, I know. WRPG style rolling dice gameplay, but some of the shots just vary so wildly I felt it ridiculously hard to play the game without VATS. There's also somewhat of a difficulty gap with some monsters. I can understand why Super Mutants should be tough, but the upper tiers of Super Mutant varieties are so damage resilient and equipped with such powerful weapons, you might suddenly find your weapon ineffective. Lastly though, the main storyline... kind of sucked. I mean, it really starts of rather weak. Sure, your character technically spent 20 years or something with ol' dad, but you as the player will have spent probably 10. MInutes. Meaning no connection whatsoever with "Dad". ...Unless. ...Unless you really, REALLY like Liam Neeson's lightly croaky, yet sexy voice caressi- ahem. Nevermind that. Back on topic. The player hardly has any incentive to go look for Dad. And yeah, that whole scene in the Jefferson Memorial about halfway through the main mission? Didn't really care about that either. Bethesda, the next RPG you make, please make the main quest interesting. :(
Really, I think the star of the show here is the exploring. I've played through a lot of the game, but I've put a lot of quests on the backburner because I just feel like walking around in the Wasteland. It's intricately detailed, and a whole lot of fun to explore. If you can look past some gameplay issues, that's enough for some people to get the game. Doesn't hurt that I got it for 18 bucks either. Not bad at all.