I bought the BMW egg from another trixter and I have to say that whilst it's not the biggest bag in the world it does hold quite a lot. I managed to get a 4 Litre carton of Oil, filter and plugs as well as some other crappola into it without any probs and still had space left.
Never experienced it sliding about side to side,not even with the 4 litre oild can in it. It's certainly the nicest bag I've seen especially when it's in small size, and it allows you to fill up with fuel without having to remove it. In the end it depends what you want from your TB. If you want loads of space then get the Baglux, loads of choices for what bag to fit to your tank cover and they do get a very good name. Me I just wanted a tank bag for carry some items around in as there's no useable underseat storage, If I want to carry lots I'll put on me panniers.
Like the man said I'm sure people will praise/flame both, me I like it, thgouh admitedly my old Oxford bag was probably more practical, but not much use on a plastic tank cover
CHeers
PAul
Keep it sticky side down