What have been your experiences with aftermarket Ham antennas on your Motorola Ht's ? Something with enough gain to out perform a stock duck but nothing that starts to severely compromise the portability of the radio in the first place.
Everyone else nailed it pretty well. Here's some stuff I can add:
Many of the after market antennas are crap, unless they are really and honestly from a known manufacturer. Chinese companies are cranking out cheap replacement antennas for many of the portable radios and really low prices. People that have put these antennas on an analyzer discover that they rarely resonate at anything even close to the advertised frequency. Basically a cheap mass produced hunk of metal wrapped in gunk.
Some even have phony brand names on them. Remember, if it seems too good to be true it probably is. If someone on e-Bay is selling lots of 10 new "Motorola" VHF antennas for $5.00, it's probably junk. Shove a paper clip in the antenna socket, chances are it'll actually work better.
Most of the bigger manufacturers will have a selection of antennas available. The last "shop" VHF portable I purchased was a Kenwood TK-5210. I ordered it with their "wide band" VHF antenna that seems to preform well across the VHF high band. I've used it on 2 meters and well up into 16xMHz range.
Still, as others said, frequency specific or sub-band specific antennas from the manufacturer will serve you well.
And I'd second the statement about "not seeing anything from the amateur manufacturers that will out perform the commercial stuff". I stopped buying the amateur grade stuff a long time ago and haven't looked back.