Howdy, Thought i'd offer a reply. I didn't have a 550 but an 81 650. I loved it.
Let me down once and it was just a dirty connection in the fuse box. Did 50000 on it and apart from routine maintenance I replaced a cam chain.
When the valves are in spec you can take it to redline if you want. Every ride I'd take it up to 10500-11000 rpm
a couple of times and it handled it well. On the xj forum the owners swear by them . Less power than the 650
but a fair bit lighter so still quick and a 6th gear aswell, plus chain drive as opposed to shaft drive i think ,but may be wrong on that one.
IMHO they are great little reliable bikes if looked after.