Yes, the gear doesn't stop working just because Sun stops supporting it. I have a bunch of old Sun and HP/Compaq gear that I keep happily humming away. Around here I am the master of old system re-use. Here are some of my rules of thumb...
1) Don't use it for anything really important. Nothing production or mission critical should be run on the old gear (unless you have a lot of redundancy (a lot!)). It's hard to explain to your boss that a major application is down because it's running on unsupported gear that just died. So, this means it's great for lab or development use. Also, that occasional software trial or proof of concept project is great for old gear, except vendors will always blame poor performance on the old gear.
2) Spares, spares, spares!!! Get as many spare drives and power supplies as you can. These are the most likely to go bad. If you put one 6320 into use, only do so if you have at least one or two identical units sitting cold on the shelf to scavenge for parts. You can't call Sun, but you can get a screw driver and dig in.
3) Advertise it! Some bosses love the fact that you are "repurposing decommissioned equipment to provide new capabilities to the department, with zero impact to the budget" (wordsmith to taste). This can give you more leverage when you do need new equipment, and it will cover your butt since management knows what you are doing (your boss can decide if they don't want to assume the risk of using unsupported gear).
I'm sure I can think of more, but these are good high points.