Yep. BioShock is a must.
The universe itself isnt as broad as the HL universe; the entire game is set in some under-water metropolis called 'Rapture'. Without spoiling it too much - it set in the 50s, you're some bloke who's just survived a plane crash into the ocean, you come across some monument in the middle of the ocean that has an elevator that leads to this city...events unfold from then on and you soon learn about Rapture's fate.
The storyline itself is brilliant and explores many themes such as power, athiesm and does very well in (disturbingly) questioning the players morals. The gameplay itself is like half life, a first person shooter but with added bio mods which give you superhuman biological powers with devastating effects (i.e. casting lightning in a pool full of people). Its also partly a survival-horror so expect a dark and uncomfortable atmosphere.
So yes its a brilliant game and my only criticism is that its abit on the short side. Nonetheless 9/10.
BUT I'd go for Mass Effect, its perhaps the best game ive played in the last 2 years and really is a sci-fi gaming masterpiece.