Oh hang on.. just read your build thread.. it was an old beater with a dented / filled top tube? Oh what the heck weld some disc tabs on it and get it out of your system!
-93 Cinder Cone with disc tabs brazed on reporting in here! Kind of concur with the previous posts - if you want to positively rip on trails you might be better served putting the money towards a used modern hardtail.
That said though, I absolutely LOVE my Kona the way it is now and wouldn't hesitate doing the whole disc tab mod again, no matter the comparatively high cost and hassle involved.
I work as a bike messenger and really fancy the ride of a 90's Kona geo around town, just wanted more dependable stopping power (i.e. not having to faff with cantis every other week to maintain performance) and not having rim brake surfaces that freeze on and off in the wet road-salty Helsinki winter... If you enjoy the retrobike ride and don't mind butchering a classic I'd say go for it - my bike feels like the best of both worlds now - but keep in mind it'll still ride like a retrobike.
If I'm going for a trail ride I mostly opt for my modern (if a 26'' steel HT can be called that) hooligan bike though...