I have had a couple of 301s, a 508, a 721 & a ViP622.
I think the 301 was the most stable- in all the years I had it, I only had one need to hard reboot it. Of course, it was also the most limited in features.
After the 301, I'd have to say the 508. Almost completely trouble free, and certainly trouble free today.
Then the ViP622. It's almost completely trouble free for me. I do get an occasional audio drop out. I love it.
The 721 brings up the rear. It's trouble free today, and I am very happy with it. I only rate it last because I was an early adopter, and it had a few quirks early on, and I'm on my 3rd one due to hard drive failures. The features that makes me prefer it greatly over the 508 are the program description with the small video window on the 721 in the EPG. This makes it a whole lot easier to pick what show to tune to.
Salti- If you're not using your 508s, might I suggest you sell them? I believe they are not 8PSK capable, and certainly aren't MPEG-4 capable. In 2 or 3 years, they'll likely be useless as programming shifts over to MPEG-4 & 8PSK. I know, there are doubts about how much SD will go MPEG-4, and how fast- but 8PSK will likely come faster. Anything to recover bandwidth. Their value must be decreasing each month. I use my 508 & 721, as well as the ViP622. I will be very sad when the day comes that the 508 & 721 are phased out.