I have the smaller one of the two sizes TT offers. The US TT importer doesn't get the bigger one . It fits my helmet and gloves plus some assorted stuff and it seems to be holding up well. No water intrusion dispite heavy rain I rode through last summer in Leadville Colorado at the HU meeting.
All in all it was worth the $$ spent.
Just my 0.02 cts
I've been using the smaller of the TT boxes for some 3 years now. I think it's excellent. Having said that I've not used any others to compare it with but have no complaints about it at all. It feels solid, is waterproof and does everything I want it to.
One of the features that I like in comparison with the 'Givi' type is that the lid is so shallow. It means that when it's open loose stuff doesn't spill out over the edge.
The Zega inner bag makes loading/unloading a doddle too.
Mine is mounted on an Al Jesse top-box cubby. I fitted the cubby originally to allow me to carry tools etc. with a TT rack on the top. When I fitted the top-box I just mounted it on the cubby.
I've just sent off for the mounting plate and will re-mount it minus the Al Jesse cubby because I find now that I don't use the storage as much as I did at first (the top-box will be where the tool roll etc. will live).
The (TT) box itself is nice but I don't like the fitting system. Mines very difficult got get off once it's been on for a few days - and I have to shove bits of cardboard between the box and the fitting plate to stop it rattling like a b*stard. That could be because I got it s/h and maybe there are some grommits or something missing, but I don't think so, I think it's just not a very good design. If I were buying again (new), I'd go Metal Mule.
I didn't have a good experience with the Zega mounting plate. It broke around the screw holes with only moderate use. TT did replace it FOC though. The stainless screws also set solid in the aluminium threaded bushes IIRC.
Eventually, I replaced it with a Hepco & Becker Gobi top box which did what it said on the tin.