I'm quite wary of DRM, and usually choose not to buy any game that has it, unless I know that it generally works (such as Valve games on Steam). I'm eyeing Shift 2 Unleashed though, and would like to know if the Solidshield DRM they use is likely to cause any problems, or if it's smooth sailing after activating.
The DRM limits the game to 5 lifetime different machine installs (e.g. install on up to 5 different machines simultaneously. Uninstall one of these, and you should be able to reinstall on yet another machine). Note that you can effectively "lose" one of these installs by losing a hard drive, re-imaging, etc. without uninstalling first.