Not sure about Bundle Stars - I've never heard of it - but the S.T.A.L.K.E.R. games are awesome. Clear Sky not so much, but each game has mods (especially Complete) that fix the problems and make them even prettier, so once modded even Clear Sky is pretty awesome - just not as awesome as the other two. Right now Steam has them for $10 (Clear Sky) & $20 (Call of Pripyat & Shadow of Chernobyl) each, and I don't think they were near that cheap even during the sale.
They are more shooters than, say, Fallout, but still have intricate and reasonably good stories and interesting characters and places. Not nearly so much as a real RPG, but very atmospheric. Artifacts (kind of alien-looking things produced by the Zone's anomalies) play a very big part - people ask you to retrieve them for money occasionally, but they can also be sold to buy supplies and better equipment, and the properties of many of them (electricity or fire or radiation protection, increased endurance or load-bearing capability, slowing of bleeding damage, etc.) are almost essential in the later stages of the game. There's a lot of content and a lot of places to explore and people to meet (or kill), factions to appease or join, quests to pursue based on leads or found clues. They are open world and you can move from zone to zone, but especially in the early stages you are constricted for your own safety.
I highly recommend them, they are some of my favorite games. If you like heavily atmospheric shooters with big open worlds, they can't be beaten.