The current A- and B-Classes are actually quite refined.

I've driven every single model (except the A200 Turbo) and all engines struck me as quiet, smooth and zippy meaning a good engine response (provided they weren't mated to the power-sapping Autotronic CVT).

The A-Class biggest shortcoming is its poor comfort. You can drive long distances in it in relative comfort but compared to other Mercedes' it's not very good in this department. On the other hand the A-Class handles very nicely for a FWD high-center-of-gravity car.

The B-Class is more comfortable and softer so overall comfort is very good compared to the A-Class. The handling is pretty bad though. All B-Classes I've driven had very light and uncommunicative steering and the suspension was just too soft. It's marketed as a Sportstourer but there isn't anything sporty about the B-Class unless you add the sports package which makes the car a little better in handling but not by much.