Aldi are currently selling an Ultrasonic cleaner.  They had them last year but I was too late!  I recall someone in the club talking about them (Dave S?) and apparently, they are very good for cleaning not only spectacles etc but also removing release oil from kits, taking off paint from bits you may have got the wrong shade on (!!!) and cleaning airbrushes.  Kiteman has one and endorses it.  £19.99 - get it while they have them - limited stock. 

http://catalog.aldi.com/emag/en_GB/print/Week5UKLeaflet/

This is the link to their leaflet, click on the right facing arrow 3 times (to go to page 6) and you can zoom in to have a look.

I bought onr last time they had them.
It does not a great deal of use.

Got to agree with Zak, the tank isn't very big so you wouldn't get much in it.Probably fine for cleaning  air-brush parts but I get on fine with a solvent cleaner if the brush is clogged with dried paint.
Nice to have if you have money to burn but for me, an expensive luxury. There's also a small risk that it could dissolve PVA if you want to clean an assembly, depends on the cleaning agent you put in. Used 'em a lot in engineering on assembly work.
Haddock.