Angus, I did use GTM at the recommendation of a very knowledgeable pal; then same pal switched to ABR ... and so have I. Do I notice anything different? I can't think I do.
Stroft (and maybe all copolymer) needs to be looked after; it deteriorates quite quickly in sunlight. Make sure that the retailer you buy from hasn't had it on a shelf for a couple of years.
It's also possible to buy it direct from the manufacturer, so you know what you're buying is "fresh"
Herzlich Willkommen - ASPO GmbH
the site's in German but it's not too difficult to navigate, and an email in English will be answered in English (not surprisingly)
Andrew