I've recently upgraded my system from MC 6.5 to MC 8, yes I know, I know, never upgrade mid-project but honestly, it's pretty simple task to roll-back to MC6.5 if I have problems.
So far so good, it's been a few days now and it feels like snappier and more stable than 6.5. I had briefly tried MC7 last year, and had had some stability and performance issues with full screen playback, and I'm happy to report that so far, MC8 has none of those issues.
For those one MC7, there's not really any compelling reason to upgrade at this point, as they haven't really introduced any new, tangible features to the software (other than licensing options) but if you buy an annual support contract before the end of the year (for $299), you'll get free version updates with it, along with latest version of Squeeze Lite, but I wouldn't bother with it, your older Squeeze still works and is more full-featured. At NAB I was assured that, while there wasn't much in the way of new features with MC8 now, by the end of the year they will have rolled out some significant updates.
Concerns: The current installer does not include Avid Log Exchange and does not support Phrase Find or Script Sync. So if those are options you need for your workflow, then you'd best stick with MC7. Of course, if you're an MC6.5 user and purchase the Upgrade/Support license, you'll have access to the MC7 installer and can continue to use it until Avid is able to eventually negotiate a deal with the companies behind Phrase Find and Script Sync. Why Avid has dumped ALE, I have no idea. It's one of their most useful tools. I tried using an older version, and for some reason even though it worked perfectly before I upgraded to MC8, it now crashes at launch. Avid has also seriously reduced the software bundled with Media Composer, but you can get it all back, along with the Symphony upgrade, for and additional $599.
A note about the new license structure: it seems that Avid has learned from the fallout around Adobe's Creative Cloud fiasco, and whether or not you get your latest version of MC via upgrade/support contract or perpetual license, it will still continue to work after your contract has expired. If you're on month-to-month, or yearly subscription it will stop working when the contract expires if you're not auto-renewing.