Electrow, regarding stability in my experience both 5.5 and 6 have been absolutely stable and reliable. If you have problems maybe there are some plug-ins that cause them or something in your preferences has gone wrong, try to start out with new ones and see if the problems persist. Performance should be the same between 5.5 and 6.5, Steinberg had done some multi-thread improvements for multi-core computers in 5.5 but since then I think no changes have been made. For a full featured DAW like Cubase performance at least on Windows is already very good and the fact that it performs worse on the Mac is certainly due to Mac OS X itself, e.g. CoreAudio and Mac OS X’s multi-thread behaviour is not as efficient as ASIO on Windows. Having said that, I’m pretty happy with Cubase’s performance even on the Mac.
The problem with plug-ins missing when opening old projects unfortunately is a downside of the ever-evolving nature of making music with computers… usually when you update a plug-in old projects won’t be able to point to the updated version as they’re looking for the old version number. It’s up to the plug-in manufacturer to provide a mechanism around that (for example Altiverb 7 automatically replaces Altiverb 6 instances in old projects). I agree that Cubase has introduced and sometimes lost quite a few plug-ins in recent versions but if you’re concerned about future compatibility of projects it’s maybe better to use third-party plug-ins that will hopefully be developed for a longer time (and which you can use also in other sequencers if you wish).
You have to decide for yourself if you find interesting the new features introduced in Cubase 6, I for myself think that 6 has been the most significant update to Cubase in recent times, there are great new features like the new comping (in 6.5 at least), way better time-stretching algorithms than before, the new way to select and modify automation and CC data (when working with virtual instruments I find this invaluable, I can’t live without this anymore), multitrack audio quantize, better transient and tempo detection, note expression, more grid options in the project window, 64 bit support on the Mac and, at least in my opinion, a far better overall look: the mixer looks more organized and professional and less toylike and you have plenty of options to adjust the GUI colours to your liking. Plus in every new version there are always some small improvements that make life easier.
All in all I think Cubase 6.5 provides a significantly more refined and mature working experience than Cubase 5.5, I wouldn’t want to go back, I absolutely agree with Norbury Brook.