The most successful was a 1.5m long mobile whip for 28 MHz. Even though it was the bottom of the solar cycle I occasionally made DX contacts with a 12 watt Johnson Viking 352D converted CB. And it worked a treat during the summer sporadic-E season.
So I have some fond memories of balcony antennas. I've since moved to a larger place with land but no balcony. So I haven't used balcony antennas recently. But if you're an apartment dweller and want to get on HF then I can recommend them. Especially on 14 MHz and up where efficiency is reasonably good.
The simplest approach is to get some sort of mobile antenna mount and screw in monoband mobile whips about 2 metres long. Buy some for your favourite bands. Add at least two radials a quarter wavelength long for each band. You do need to swap whips to change bands but at least you're on the air. And, despite compromises with the whip length being significantly shorter than full size, you can get some good results especially if your balcony is in the clear and/or faces a useful direction.
Here's some accounts of other peoples' success with balcony whips on various HF bands:
* Long Reddit thread on balcony antennas
* UR5WCA balcony antenna for 7, 10 & 14 MHz
* Ham Radio on a very short antenna
* Buddistick balcony antenna (video)
Have you had success with balcony antennas? Or maybe some failures? Either way, please share your experiences below.