I've been a big Tomb Raider fan since the very beginning, and I'm glad that after those last few developed by Core (circa Last Revelation to around Angel of Darkness) Crystal Dynamics took over and really breathed life into what frighteningly looked like a dying franchise.
TR: Underworld doesn't disappoint. The visuals are certainly the best yet in the series, where owners of high-end PCs will relish the clarity and detail of the surroundings, which provide a colourful and refreshing breath of fresh air if you've been playing dreary first-person shooters quite a bit like I have. The gameplay is also a lot tighter, the latest incarnation of Lara having an expanded array of skills, the movement of which is very much more human and fluid thanks to the choreography and motion capture of gymnast Alison Carroll. Combat has also been tweaked and is certainly more dynamic, though some people would argue that the inclusion of the so-called "Adrenaline Mode" still makes things a bit too easy, so its strange that they went to so much work to make combat more of a central theme in Underworld, and yet still haven't made it as challenging as some of the revamped platforming elements that the series is better known for. Additionally the camera still has the habit of producing some nausea-inducing shaking and bugging when you're in certain close spaces, not to mention having a will of its own when you're trying to get a better look at something.
There are also occasional instances of clipping, where Lara seems to be able to stutter her way through some small obstacles, getting caught on some rocks which cause a glitchy looping of movement and visually loosing a limb inside a wall or floor. From the experience I've had playing it, its more a detraction from the atmosphere of the game rather than an encumberance that prevents you from progressing, so it shouldn't count against the decision to buy it or not.
This might sound like a lot of negativity, with positivity only focused on graphics, but this isn't my intention. If I had to put it simply, we've often found ourselves battling glitches like this with previous Tomb Raider games, but it obviously hasn't put off the huge fan base, and quite rightly. Its still a hugely enjoyable game that delivers what you'd expect from the series. So if you liked Tomb Raider Legend and Anniversary, you'll love this.