I've just been getting acquainted with Rhapsody of Fire. It's not half bad. The singer isn't quite my cup of tea, but he's not particularly annoying either. (Like I've said earlier, the vast majority of metal for me is ruined by vocalists I don't care for. Well, the same applies to any genre really.)
Now, this type of music seems to rely heavily on the idea of the concept album. Rhapsody of Fire has taken this to the extreme by creating an epic fantasy saga spanning multiple albums. This has its pros and cons. The idea of a continuing saga is pretty cool, but it means you really need to listen to the entire work as a whole, and it is harder to really get into.
Nightwish, on the other hand, while it can be seen as working in the same genre, symphonic power metal, doesn't do concept albums (or at least such themes are much more subtle). Songs are complete in themselves, which means they hook you in a very different fashion. It was a single song by Nightwish that got me interested in them, and the whole symphonic metal genre. And after that single some came another, and another, and another... great, anthemic songs you can just listen to over and over.
Whether it has something to do with the different approach to songwriting, or simply a result of different personalities and songwriting skills (not to mention the very different style of vocals), it's hard to say, but I don't think this type of music can ever have quite the same appeal to me that Nightwish, for example, has. The same applies to most concept albums, in all genres, not just Rhapsody of Fire and other metal albums. Take Aimee Mann's The Forgotten Arm, for example. While there's much to speak for it, it just doesn't hook me in the same way that the albums before and after it do.