Before a CBT, most training companies do a taster session. Roughly an hour on either a moped or a geared bike, which gives a good idea how riding is.
The CBT will get you on the real road to learning to ride. All weather conditions, all traffic conditions. Most modern 125's will reach 70+, but they don't have the grunt to get out of some sticky situations you might (and most likely will) get yourself into.
I've had my 125 sine June and wouldn't fault it. I'm due to do my DAS in June, but I will not be looking at anything much bigger than a 700 for the first however long. It's mostly for commuting but on a nice sunny day it's hard to beat the feeling.
Fish