The drive from Bangalore to Coimbatore was simply excellent. We opted for the Krishnagiri-Salem-Coimbatore route. About 320+ kms and took us all of 5 hours. We started early from Bangalore at about 5 a.m. and at a comfortable pace we arrived at our hotel in Coimbatore by 11 a.m. (I'm told most can reach Coimbatore much quicker but we took longer; this with a couple of short break stops along the way).
ROADS: The big plus is the roadways. Highway most of the way and very well kempt roads. We had absolutely no trouble at all and the drive was extremely peaceful. Also maybe we lucked out on a holiday because it wasn't very crowded and the roads were quite a delight.
VIEW: Also I never knew this but the roads to Coimbatore offered some scenic views. All this put together, ensured us a pretty good drive.
I'd definitely suggest driving down to taking an overnight public transport because there is much to see and experience enroute Coimbatore. Also plenty restaurant and snack stores you can stop for quick breaks.