Like you I had several legacy projects left over from CS4
Apart from obvious 64bit third party software upgrades needed to work with CS5 I found the transition pretty seamless. Only Boris Graffiti has not been upgraded to 64bit which is a shame as I made considerable use of it.
I got round the bloating problem before it ever occurred – by starting a new project with a CS5 suffix and importing all the sequences into Premiere. I also made sure that all the software upgrades worked as they should before finally disregarding my CS4 cuts.
There are good reasons for upgrading your projects, not simply waiting to finish them on CS4. The first is that you can take advantage of CUDA enabled MPE (presuming you have a compatible card) for GFX acceleration. Even in non MPE mode it is significantly faster and really does make a fantastic difference.
Secondly – CS4 is a complete dogs dinner in terms of resource usage and it was an absolute pleasure to leave it behind for a proper integrated 64bit architecture.
Thirdly, you can visit the Cow forum anytime now and not feel you’re being left behind as more and more issues deal with CS5. (not crucial I admit.)
The downside is that you have to reinvest in upgraded software in one hit for plug-ins. Ce la vie!
My conclusions: few issues if any in the move up to CS5.