I used to own a stock BD-2 which I actually thought wasn't a bad pedal. I was never that big on BOSS overdrives except I always felt the BD-2 was actually good.
The Keeley mod is well worth it though. Direct differences is that the Keeley has a slightly more open feel, with a bit more depth. Great little switch to add more bottom end, which I always have on... sounds huge. Also, when you really crank the gain on this pedal it sounds very early sabbath sounding which is quite surprising. The gain range is huge - great for simple boosts, to "on the edge of breakup", right through to early sabbath esque gain - almost fuzz sounding on max, with the added bonus of the bass switch... and, it doesn't really colour the original amp sound either.. it does a bit, but nothing like a mid bump like a TS if you know what I mean. Great little pedal. I see them on ebay now and again. I'm still tempted to purchase a second to use side by side.