I've got a Lomo LC-A and a Holga but not a Diana ... yet
The lomo is much easier to use and is quite a robust camera. It has auto setting and 4 distance settings (0.8m, 1.5m, 3m, infinity). ISO ratings up to 400. You just leave it on auto and shoot away. Results are great, if you like that sort of thing, v slight fish-eye effect on the lens and quite heavy vignetting (with a good exposure). You should be able to pick one up second hand for £50.
Holga's are a bit more of a 'challenge' to use

One shutter spead, two apertures (f11 or f16) urm ... and that's it. Manual focus lens which is all guess work as no distances are listed. Some come with flashes, some come with glass lenses, mines plastic. Severe fall off of focus and heavy vignetting with my plastic lens. Back tend to pop off with film in it. Botton on the bottom which can be switched for 'bulb mode' is always getting accidently turned on so tape all over the camera. Subject to light leaks. Can be got for around £20 to £30 depending on the model.
I dont have a Diana but from what I've seen it seems to be a bit more extreme than the Holga.
I much prefer the Holga results to the lomo results but I get a much higher hit rate with the Lomo. All three cameras produce quite different results which have loads of character. If you can nail a Holga shot the results are just beautiful.
Either way they're both lots of fun to use and I'm finding myself using them more than my 30D these days.
I'd suggest buying a lomo first as it's easier to use and easier to get film developed. Pay no more than £50 for one off an autions site.