I put up a loop a couple of months ago. I planned it to be a full wave on 80, with the wire running down the second floor eaves on both sides of the house and out to trees in the front and back yards. A big wind storm the next day (80 mph gusts near us) broke it, so I re-ran it around the eaves but only to a tree in the back yard. Now my MFJ analyzer says the fundamental is about 5 MHZ - a band I have no plans to ever use - but my Z11 autotuner tunes it well on 10 through 40 meters. I didn't have some RG8x handy, so I fed it with (get this!) 24 gauge clear-jacketed chee-e-ep speaker wire I had lying around. I mentioned it on eHam.net and a couple of the experts said it would be lossy because of the bad feedline, but .. you know what? Though performance was generally not as good as my Butternut HF9v or my triband vertical dipole at 20', in the 10 meter contest it outdid both the other antennas in every direction except West. In those directions it was 1-2 S units better than the vertical dipole (my best 10-15-20 antenna)! I ended up using it for most of the contest, switching back and forth frequently to check it out. It consistently did great.
Loops are said to have a low angle radiation when they are at least a 1/4 wave up and 2 or more wavelengths long, and my experience proves it. Next I hope to rework the loop to increase the length and bring the resonance down to 3.5 MHz or lower (and I will probably change the feedline to RG8x, as I had planned - I do believe the 24 gauge speaker wire is probably lossy).
For additional info, I run only 5 watts on HF, but I was making 28 QSOs/hour searching and pouncing in the 10M test, and racked up 120 contacts in a few operating stints over the weekend - big fun!
Bottom line - do the loop. I think you will be pleased with it. Just make it as long as you can (enclosing the maximum area possible - that's important) and get it as high as possible. It may not be a beam, but if nothing else it's a great backup antenna.
I have continued to use the loop along with my verticals. The only place it works best is on 10M. On all the other bands it is probably a "sky burner", but I just don't get as much signal off of it as I do the verticals no matter what I do. You'd think I would find some signals stronger with the loop (the closer-in ones) and other signals stronger on a vertical, but it seems like the verticals are just better all around anywhere except 10M.
Because the SWR on my HF9v is a bit high on 30M I was using the loop there, but one night happened to switch to the high-swr vertical and noticed that a station I could hardly hear on the loop jumped right up out of the noise. So, since SWR is an indicator of impedance match and not radiating efficiency, I have gone back to using the vertical for 30M, high swr (maybe as bad as 5:1) and all, and it definitely works better.
Maybe the loop is lossy because it's so low - only 18-20 feet up, running around the second floor rain gutters - or maybe it's how close it is to the house. I put it in some plastic clips to hold it off of the gutters, but they are cheap and weak and the wire (an insulated enameled wire, tho) just rests on the gutters in places. Maybe the house just makes it lossy. OR maybe it's because I am feeding it with 24 gauge clear-jacketed speaker wire.
I don't know, but am thinking again of devising a way to hang a new loop higher in the trees (it might slope, or be irregular in shape and height). Any antenna works better "in the clear", right? Feeding it with coax might make a significant difference, too. (Maybe I should see if I can get my HF9v vertical up on a higher mast ...)
As it is, the loop is better than nothing - it makes a good backup antenna and, with the cute little Z-11 autotuner, always presents an swr <1.5:1. And, of course, it is really invisible, even to me unless I'm a few feet from it. 18 gauge enamel wire is great stuff for stealth work! And the loop DOES work, though 20' higher might make it really sing! I hope to get a chance to try that ...