Descent is propably my favourite game series ever. And this here is - unlike many people say - not at all a bad entry.
Descent 3 takes a different approach then its predecessors, by having a mission-based storyline rather then a huge amount of Levels that all share the same objectives: finding all keys, blowing up the reactor/boss robot and reaching the exit in time. Not to say that Descent 1 & 2´s approach was bad (not at all. I loved it), but a third game with the exact same style would not have been a good thing.
So what´s so great about Descent 3? For me it definitly is the level-design. Every level in this game feels unique and compelling. You´ll have to fly your way through vast, wide open areas as well as narrow tunnel systems that are all very well build and structured. Everything is just beautifully connected and all parts of an area ultimatly form one whole, believable, almost "realsitic" complex. This is even more true for the excellent Levels of the expansion pack. Make sure to check it out, it´s worth every second.
Since Descent is mainly a 3D action-shooter that is about blowing up flying robots, you will - naturally for the series - find a huge amount of weapons at your disposal, including all kinds of lasers, gatling guns, rockets and explosives. The diffenece between each of these weapons is not only their power and ammo type/consumption, but that they are like counters to the different enemies you´ll have to fight.
For example there is a weapon that will shoot right through your foes, hitting multiple targets in a row (even when the first one is not outright destroyed) which is perfect against small, swarming enemies that are usually clumped up. Or there is a sniper-like weapon which is great at taking out thougher targets from a distance. There is even a weapon that leeches health from your enemies adding it to yours. Rockets can either simply explode or have special effects like bursting into a shower of shrapnel or napalm, heat seeking energy balls or even more heat seeking mini-rockets. And absolutly nothing beats the satisfaction of a well placed "black hole-ish" black shark rocket sucking up whole armies of enemies and destroying them in one huge explosion!
The list goes on and on, but you´ll have to play it yourself to fully understand ;)
Last but not least I´d like to mention, that I realy enjoyed Descents 3 storyline, especially when looked upon as a whole (including the expansion pack) and it was a nice and well deserved ending to a great game-series. I actually found this to be astonishing, since the frist 2 games did not realy pay much attetion to storytelling, but the third installment realy felshes it out and rounds it up.
All in all I highly recommend this game not only to fans of the series but to everyone who wants to have a truly unique, refreshing and classic action shooter experience from a series that in my opinion hasn´t been matched yet.