Only worth it for longtime loyal fans to the series, and even then -- barely.
What Underworld does do better than all its predecessors is look good. The graphics are just oozing with detail and realism, from the pores on Lara's skin to the glossy sheen of wet stone. And not only does Lara have some nifty new animations to show off, she does them with a highly convincing physicality and weight. Pro diver, marksman, kick boxer, gymnast, and historian? No problem.
Unfortunately, just about everything else is lacklustre, simply because of the sense that the developers put this game out in a hurry. The whole thing can be completed in probably around 5 hours, and this time around there's no Croft Manor to bide your time in outside of the main levels. I took longer because I'm a compulsive completionist who scours levels for relics and treasures -- at least, I was for a while in the beginning of the game, but after a while I simply lost interest and ended up ignoring the treasures completely. Why? Because I eventually realized there was no real point to them, other than getting to see some concept art (which has likely been posted online somewhere, awaiting Googling). Treasures are scattered so liberally that you can't walk ten feet without tripping over one, and finding them feels less like a discovery and more like finding the socks you lost in the laundry. When you pick one up, you hear that fun "bliiiiing!" jingle from the Classic games, but there are so many of them that the novetly of it wears off fast. The impression I got was that the treasures were thrown in at the last minute, and placed anywhere that seemed a little too empty to the designers.
I discuss the treasures at length because frankly, there's not much else to do other than solve the main puzzles and kill occasional enemies. Combat has always been a vice of the series, and here is no exception. Your enemies range from stupid wildcats, stupid marine life, stupid men with guns, and stupid undead creatures. Oh, and really annoying bats and spiders. Those are literally the only foes you face, and with the endgame weapon, they are laughable trifles. Admittedly, it's funny to watch Lara kick a tiger in the face Saxton Hale style, but only just. Otherwise, fighting is nothing but a mindless button mash that's over before you have time to realize what you're shooting.
Speaking of puzzles: well, the ones in this game aren't terrible. They just consist of every kind of puzzle you've seen before in other Tomb Raider games (think light-and-mirror puzzles, weights-on-buttons puzzles, and sliding-doors puzzles. More than once I had a vague sense of deja vu, and I found myself identifying the puzzles not by using my brain, but by scouring my memory of past Tomb Raider games.
The only new features that I can think of are a) a totally useless "prep" stage before each mission where you choose which weapon you want to blindly kill things with, and in which skintight outfit to do the killing in; b) a storyline that failed to engage me, even with the death of a major character; and c) camera issues that somehow manage to be even worse than before. I found myself clambering over the same rocky walls again and again because of the spastic contextual camera. That is, if you move the mouse a hairline to the left, that means you won't jump to the ledge behind you but instead into the pit full of spikes to your left. For a large portion of this game, I was fighting with the controls instead of using them to my advantage; no amount of fancy new flips and spins can solve a core design problem.
And on a final, relatively minor note, I found the soundtrack at best unremarkable, and at worst somewhat irritating. Essentially it is the first few notes of Wagner's "Ride of the Valkyries" watered down and played over and over again, except that instead of sounding like a tomb-raiding adventure it sounds kind of like a bizarre sea shanty. But maybe that's just me.
All in all, Underworld is a rushed, humorless, and weaker shadow of the previous games in the series, filled with all the same problems and adding a few of its own. The puzzles are simplistic, cliche, and without replay value. Combat is silly, and not in a good way. And technical aspects of the gameplay are as bad, if not worse, than before. But hey -- at least the graphics are great.