I downgraded from a BlackBerry Bold (which is supposed to be a more superior handset) to the BlackBerry Curve 9320 - and I am extremely happy. It's one of the best decisions I've ever made.
It's a rock-solid phone in terms of stability and it's also relatively quick (the navigation and the use of applications).
My only complaint is that the default Browser application is slow and sometimes crashes when too many (2 or 3) tabs are open, with websites that have many images or lots of content. Besides that though, the BlackBerry Curve 9320 works very well. I haven't rebooted mine for 2 weeks now. It keeps going...
However, one trick I've learnt with the BlackBerry devices: Do not install too many apps on it. The more apps you have, the worse it becomes.
I've kept the phone as stock-standard as possible. So default software with these apps only: Twitter and WhatsApp
NOTHING else!! No crap weather apps or anything else. Too many apps kill the BlackBerry in my opinion.
I think you should go for it, especially if you loved your previous Curve 8520 - it's everything like that and better.