Ubuntu 9.10 64-bit here.
Pros: Everything tends to work.
Cons: Until an upgrade/update occurs, then who knows?
I've never had a problem with an upgrade/update doing anything with my Arduino IDE install, but I have seen a full upgrade get jiggy with my installs of Wildfire (Spark IM server) and ZoneMinder (security camera software).
The only other con, at least moving from 9.04 to 9.10, was that somebody was smoking crack when they decided on the "new" color scheme. I don't know if the latest version has a similar scheme, but this version's was UGLY.
Oh - the only thing I really, really hate: Whether it is Ubuntu or the GNOME team (probably the latter), they seem to think "we know better than you" when it comes to what options and settings you want - for instance, it is nearly impossible to set the configuration for a particular screensaver; they've removed that and have no intent of allowing you to have it back. There are ways around this, but I shouldn't have to hack my way around something so basic.
Part of me thinks I should move back to Debian.