I have a few issues with 5.5, but they’re mostly visual things like the markers look crappy, I can’t read the marker name, etc. But there are so many positives that I wish I could run it all the time. My PC, however, is still running Windows XP SP3. Under that, Nuendo 5.1 is rock solid, so i use that for my day-to-day. On my Mac running OSX 10.75, Nuendo 5.5.5 is great, and 5.1 is not. On my other PC running Windows 7, version 5.5 AND 5.1 run great.
Oy, so many things to think about 
I personally feel Steinberg needs to recommend at least Windows 7 for version 5.5 and up.
I don’t know about Nuendo 6, but I assume it will run best under 64 bit, which is where everybody seems to be headed. My Mac is not able to upgrade to 10.8 - will I be able to run Nuendo 6?
Thanks. Note to lalithkr: It’s true - ANY DAW software at one time or another is going to run like a POS. Just the way things are, depending on plugins, OS, computer, time of day, etc. I just try to find whatever version runs best on that machine and NOT upgrade it, but rather upgrade the computer. Sort of like if you’re 60 years old, you might be able to get a 20 year old girl to go out with you, but know that you’re going to die in the act of trying to perform like a 20 year old guy