I just finished TRU, although with the DLC levels coming, it's not quite over just yet. The wait shouldn't be too long; it looks like it will be available next week.
Underworld felt like a longer game than Legend, but only by a little bit. The last two levels in the game were pretty short (really just one level; the second to last level is an extremely short revisit to a previous area just to trigger a cut scene); echoing the final two levels in Tomb Raider Legend. Underworld also does a nice job of closing the story line begun in the last game, tying up all the loose ends not only from Legend, but Anniversary as well.
The levels were all fantastic and fun to play through, and thankfully the motorcycle's return was actually integrated well this time around, unlike in Legend. The visuals were also top-notch throughout the game, although the camera had problems; dying because you missed a jump or couldn't see where you jumping to is not enjoyable, and it happened a little too much in this game. Not enough to be frustrating, but still annoying all the same.
Overall I'd recommend Underworld despite its camera and technical issues. Add to that a disgruntled ex-Crystal Dynamics developer spilling the beans that the DLC was meant to be part of the final game, and it's clear this game was rushed. However, it was easily one of my favorite games from 2008 and definitely worth playing if you liked Legend and/or Anniversary. If you haven't played Legend yet, this game is a direct continuation of its story, and it's worth playing through Legend before Underworld.
There's talk of this possibly being Crystal Dynamics' last Tomb Raider game, since sales fell well short of expectations. The series might be in for another developer change and complete overhaul once again. If that's the case, I have highly enjoyed Crystal Dynamics' run on the series (actually I need to finish Anniversary-- only played through the first four levels!); they did a far, far better job on Tomb Raider than Core Design ever did.