I've been using waves RVerb alot over the last couple months. I just tried out ReverbX (thanks to tubedude's endorsement). Seems like it is a little less versatile, but it sounds very good. It has a warm, sweet sound. The waves sound, by comparison, is a bit colder. I'd also say that waves Rverb is a bit more trasparent, while reverbX leaves more of a mark. Of course, this could just be a factor of my configuration. I particularly like 'warm plate' reverbX. On reverbX, I find that even when I have the reverb set too high, instead of flinching at an unatural sound, it is actualy pleasant to listen to.
On my dual MP1600, at 24bit 96K, I can just barely run reverbX with the Hall, Room, and Auditorium algorithms. It's the biggest CPU hog I've seen to date.