I bought an L1 a week or two back, but have only just had the opportunity to run it other than just as a test. Like one or two others, I found a slight tendency to derail at certain points. I have had the same problem on one of the LMS 2-6-4 tanks on which, because I had bought 2 and one was fine the other not, I checked out the differences and found the back-to-back on the derailing loco was too tight. I've done the same test on the L1, opened out the wheels slightly, added a bit of lead on the leading truck, and it's now fine.
I don't really like this design of pony truck as it seems very sensitive to the back-to-back measurement and it takes up a slightly odd position to the rails on tightish curves - I think the effective pivot centre is slightly too far forward. On the other hand, it works when properly adjusted, and the lack of springing does improve adhesion. A bit of extra weight on unsprung Hornby bogies/pony trucks doesn't hurt.
The L1 is an impressive model, and it pulls more than I expected too. I've just tested it on an 8 coach kit built and weighted set, and it pulled away with prototypical slipping on a slightly rising gradient.
Colin