Are you on about SiN the original game? Or SiN: Episode One?
If you have not played the original SiN, then it's probably worth buying SiN: Episode One from Steam, as you get the original SiN game with it. It's a bit old, but it's still a laugh, and shows what Half-Life was really up against at the time.
SiN: Episodes isn't really that bad, but it's very samey, it's quite a good enjoyable action sequences, but you're limited on weapons and enemies. The whole package though for £13 isn't that bad thogh really.