[Captain Slow]

Probobly just reinforceing whats been said so far but.
DEATHWATCH.
drawn from various chapter that sumbit themselves to a commity. the recurits drawn from those chapter are quickly taught the meaning of a hard time. the deathwatch do not perform suicide missions. however the missions they do execute are almsot that. in serveral books, warrior coven, and the graphic novel deathwatch (awesome book) the idea that they perform the task that not even a normal space marine chapter is asked to perform is reinforced in these books/graphic novels. as well as the fact that, whiole space marines, not all of them come back alive. thier term of service is not a set thing merly the amount of missons they must perfrom. i dont know quite how it is judged, however in various fluff ive read thsoe that do come back alive, have completed 2 maybe missions with any one Deathwatch captain/Team leader.

the idea that they simply fight the Tyrandis is a flawed one. while in many popualr pices of fluff they are deoitced or described as battleing tyranids, it is described in thier GW fluff as, they are the last line of defence against Xenos. so, while not mentioned, they no doubt battle a myriad of other alien races. TAU and Orks foremost amoung them. espically tau, sneaky gits...ahem.

as for game terms the deathwatch are very nasty. theres tacticus such as simply fielding a whole squad of them, just with heavy bolters and suspensors and trgeting things like power armoured nobs, and carnifexs, as thier hellfire rounds cause a Small blast that auto wounds on a +2 something ive used against a hore of death guard terminators, they were cut down in droves.
as for the rest of the upgrades, normal SM's are not invicble, and do make great close combat attackers, even with thier normal boltguns, however the pissiblity to outfit an entire deathwatch squad with veterans and power weapons and then to top it off a librian, while expensive, in large games, and in APOC sized games makes for a very powerfull shock troop unit. all in all, the rules do reflect the true nature of Deathwatch in comparsion to normal SM's.

anyway, hope that helps.

(on a side note, after RE-reading Deathwatch so i could accurately portray the DW in this post, im tempted to do a squad based on the DW in the Graphic novel Deathwatch. well, after the first of my fists are out the way at any case. ^^ )