I was running 10.4.11 and decided to upgrade to 10.5.8 last night. I just stacked OSes because I didn't feel like backing anything up. So far, no issues. Thinking I may give 10.6 a shot in a couple of days.

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Jeff
Current:
'07 Jeep Wrangler
'06 BMW X5 3.0i

Former:
'12 WRX; '08 WRX; '06 BMW X5 4.8is Dinan S1; '02 BMW 530i Dinan S2