I bought a 56 plate YBR125 when I passed my CBT to get used to commuting into London with gears prior to taking/passing my DAS 8 months or so later.
Ideally I wanted a Honda Varadero 125, but these are incredibly expensive for a 125, so not worth it unless you plan to keep for a long time. Also considered Honda's CG125, however, as these are no longer made, there were hard to come by + prices were over-inflated.
Firstly, worth noting that I'm relatively large for a bike of this size (6'4" +16.5/17stone) and my YBR was the carb model, not the later fuel injection one. Predictably, I looked like a toad sitting on a matchbox when riding it, but what was more of an issue was the lack of power. Not sure how much this was down to my size, but the YBR was slower off from the lights than everything bar London cabs and pizza delivery bikes!
Whilst you don't buy a 125 for its speed, I found the lack of acceleration got me in a few sticky situations where I didn't have the grunt to get out of the way of tight spots created by some of London's less competent drivers! I also never managed to get the bike over 50mph (my 50cc Aprillia scooter from 10+ years ago did 60mph!).
Also, as others have mentioned, the very narrow wheels/tires and unsubstantial suspension for my weight made the bike very unstable over bad road surfaces and pot holes. This, coupled with the bike's light weight, made it pretty hard to hold on to in strong cross winds too.
Having said all of the above, the YBR was peanuts to insure, would do 90mpg around town and never really let me down. Maintenance-wise, it just needed a couple of oil changes and regular drive chain adjustment.
Finally, if you're going to by a YBR, make sure you take it out for a long test ride to check the clutch operation. Many of them have a common idiosyncrasy where the clutch cable (which goes over the engine) stretches when warm causing a snatchy take off in 1st and 2nd gear. Although I replaced the cable and frequently lubed and adjusted it, this issue was never fully rectified. Again, this issue may have been exacerbated by my weight.
In summary, I would say that this bike is fine as a stopgap between CBT and DAS or if you’re a lot lighter than me. They also don’t seem to lose value – I practically got the money I spent buying it from a dealer back when trading it in against a Yamaha MT-03. I wouldn’t, however, recommend this bike if you’re large and planning to ride it for a long time – if this is how you’re intending on using your 125, it’s probably worth stumping up the extra cash for a Varadero.