We'll I've had my SSG 5 for about 2.5 years now and it has worked great, and will probably keep working for many years to come. But Juniper have released a new/replacement model (kind of, they're still selling the SSGs) that runs JunOS.
So I thought it was about time to learn the operating system as ScreenOS (OS on the SSG) will eventually be discontinued.
The SRX 210 is really more of a replacement to the SSG 20, but it looks there isn't a SSG 5 replacement (yet at least, I did see some mentions of an SRX 100).
Anyway hopefully I should get it next week and then I'll do a review of it.