I also looked for a cheap macro hobby solution. I fact I already had a 50 mm Pentacon f1.8 Auto lens from my old manual Practica LTL-3 camera and original set of Pentacon extension tubes. So I just bought a cheap 49 mm screw-mount to K-bayonet reduction for my Pentax K-x.
It works, but is has all these disadvantages as mentioned (probably the same as the reverse mount): you have to go very close (depends on the extension tube used - in the set you have 3 different lenghts), very shallow depth of field and also the extension tube decreases the number of light going to the camera sensor.
I also have to manually push the F-stop button on the lens. This lens is called auto because there is also a small pin, which is used to activate the aperture by the camera before taking photo, however in this case it cannot be used and you have to activate it manually.
There are in my opinion two important things: first, has the reversely mounted lens manual aperture control? Second, do you use reverse mount on other lens ((I saw someone who had a telephoto lens reversely mounted on the standard lens) or directly on the camera?
In fact, if I use the shortest extension tube and take photo of some flowers when no wind moves them and I have a plenty of sunlight, I can achieve relatively good photos (as the lens is pretty sharp and has almost no aberation etc.) using the Pentax focus assist function - just press the shutter release and then move forward and back and camera takes the photo when the focus is detected to be OK. But needs a plenty of patience.I also have Tamron 70-300 with "macro" and it is much easier to operate. In future I plan to buy something like Tamron 90 mm macro. :-)