fairly sure it's *only* true if the program loaded kicks the fastloading routines out of memory (though I would assume this would be at least 50/50)(though its probably worth noting that with the C64 especially, you have to reboot the system to load a new program about 99.9+% of the time, particularly with commercial software...which then 'reloads' the fastloader anyways)
(note: the bottom of the linked comparision page does mention this:
Cartridges typically do not speed up sequential write/read (actually, this refers to CHRIN/ACPTR/CHROUT/CIOUT
kernal routines used mostly by multiload games and applications) but often feature quite fast LOAD routines)
since JiffyDos is actually replacing the original commodore DOS it isn't affected in quite the same way as a cart which boots routines into ram (or of course software speedloaders which have to be loaded every time you want to load a program generally, and aren't a lot of use)
I have owned Epyx Fastload, FCIII and Mach128 carts and all have worked acceptably well, I also have a BetterWorking Turbo Load and Save, but i haven't really bothered with it (I think it is probably on a par with the Epyx FastLoad)
Compatibility is also very high with JiffyDos (some programs fail to load at all in some cases, but these are usually programs with thier own software fastloaders built in(ie: GEOS)), as it is with the Epyx Fastload and the FC series, though I seem to recall a few more compatibility issues.
I think the only 'mainstream' fastloader I have never really had a chance to use were the WarpSpeed ones.
one of the
real advantages of JiffyDos being that is it still commercially available and you can just order it.... (tho i gather a few guys have done 'clones' of some of the fastload carts)
here is a bit of a comparison between various fastloaders for the C64:
http://www.c64-wiki.com/index.php/Comparison_of_fast_loaders
and a list of (some of) the various fastloaders:
http://www.c64-wiki.com/index.php/Fast_loader