some people have problems with the initial start up of the FX5. if not assembled and primed properly, it wont run properly. (much like anything else). as with most user error, its easily set right. if you do get one, make sure to read and follow all set up and assembly instructions, and you should be fine.
aquaclear HOB. sorry i know nothing about this. you think that would be better then the fx5 ? ill do a google search on the aquaclear now.
it depends where you are located. hagen UK does not carry the aquaclear line. (though nobody at hagen uk could give me a reason why. even they dont know. hahahah)
as far as fx5 vs. ac110... thats a tough one. i like both filters, however my oldest aquaclear110 HOB is 8 or 9 years old, and has never given me any problems that weren't user errors. and i have personally spoken to people who have been running thei aquaclear500's (the older name for the now aquaclear110) for 15, and even 20 years. 20 years out of one filter is pretty darn impressive no matter what brand you prefer.
based on cost, i would rather run 2 ac110 rather then a fx5. but i do also run the fx5 canister on some of my tanks.