In the late eighties/early nineties the USCF (I think) ran a series of
tests of tubular glues, including Fastak.
The test results are at:
http://www.engr.ukans.edu/%7Ektl/bicycle/bicycle.html
As I recall, the conclusion was that the best glue at the time was
Vittoria Mastik One, followed by Continental.
Fastak worked best if used on preglued rims & tyres, and relatively
poorly if used on its own.
Since that time, Conti have changed the formulation of their glue,
apparently to bring it line with EU environmental regulations. Though my
view may be clouded by the fact that I sell the stuff, my impression is
that the new formula glue is a superior product. I glue a lot of track
tubulars, and it provides a firm & durable joint that can't be broken
without some extra leverage.