I've been using the SIMBL hack since it first came out and I haven't experienced any stability issues with the OS nor heard or read any complaints about it. The colorfulsidebar.bundle has been updated for ML, and so far I haven't experienced any issues with that either. If anyone is using Lion or ML. There is nice little bundle on MacUpdate that has the colorfulsidebar.bundle, SIMBL, the relaunch script, and a injector. Once you have the RelaunchFinder app in your login items it only adds a second or two to your login time.
ML is what SL was for Leopard. What I mean by that is SL just made the OS more stable, faster, few more features, and improved upon features Leopard had. ML is just the same. I think it's worth upgrading for SL users that have the multitouch MacBooks and recentish Macs. A major anoyance with ML is they changed the Dock's files so you can't use any customized Dock skins others have made. That might not be a major issue for some but it is for those who like to customize. There are some minor bugs with ML, but from my day 1 experience it has been far more stable than Lion was on day 1. Another annoyance to some is you need to have a pretty recent, like a year old, MacBook Pro to use the Airplay feature and Powersleep or w/e it's called feature. But if you really don't care about those features then I think it's still worth updating to.
As for customizing, the developer of Dock Library hasn't updated the app for ML. The developers of Candybar have bassically given up making the app free, and only updating when need to. You can still customize app icons and system icons so Candybar isn't completely useless. You can still use Mirage if you want to hide the Dock's border.