I've got the F5, and it's a great machine once you learn to use it. No complaints about build quality, either. I've asked numerous people who claim that the build quality isn't as good as some other brands, and they can never seem to give me anything beyond "it just feels cheaper". Oh, really? sounds like they're just broken records of what they've been told about the brand since changing ownership.
The F5 is a great coin/relic machine and is pretty light-weight. It goes pretty deep (I've found wheats at ~8"), and the ID is pretty much always right. If it isn't right, it's because I didn't interpret the readout correctly (i.e. shaky 68-72 for dime coming out as a copper penny; a real dime will be a lot more steady 69-70, but that's me not having learned the machine enough yet).