You pay your money and take your chance. Personally I would stick with XP until Vista has been out a bit and the problems are solved. There will be problems, creating a new operating system is a massive undertaking and some programs may not run properly as well as some items of hardware not having drivers yet. To be fair to Microsoft its not possible for them to test it works with every bit of software out there are theres just too many variables.
It would in my opinion, if you plan to get Vista, be sensible to buy a PC that was offering a free Vista upgrade as a lot of the manufacturers are doing.