First rule of thumb: If it doesn't have staying power (see UT2k4 and UT99- they've still got quite a few active servers going...UT3 is like a ghost town in comparison...) then there's something to it and it wasn't worth buying.
Second rule of thumb: If it doesn't have an actual native binary available for Linux in a supported or unsupported fashion you probably ought not to buy it. Not even on "promises" to do a version. I got bit myself with NWN doing that. Even though they carried through, it was a flawed result when it came out (No cutscenes...some cross-platform title that was... ). So I told myself no more. You should too, especially if you got bit on this little debacle. If it's not got support for it at the time of purchase, you probably ought not to even consider it unless that purchase seals the deal with <X> prepurchases meaning that it will be actually done in a timely manner (See: Ankh and Jack Keene...).