Minus 27C64's... I had nothing but trouble with my Willem... I had a special ritual I had to follow to burn pretty much anything else... low voltage, adjust timing... medium voltage, then hammer it home with higher voltage... that's the only way I could really get 2532's and the like to burn... if at all.
Picked up an Needham EMP-20, haven't looked back.
As for a UV eraser, I got one of the cheap ones on eBay, it's worked well enough for me, but now that the bulb is going out, I won't be replacing it. I think I'm just going to strap a couple of bulbs to the top of a shoebox and use it instead... When I order my EPROMs they're never blanked and having to to through, sometimes, 100+ EPROMs 5-6 at a time... ugggg. It just depends on what you need though.
Willems seem to work for some people, sometimes they don't. If you just need a couple of EPROMs burnt or read (Willem did a fine job of reading the chips, just shotty programing them) then a Williem might work for you... but I'd recommend against one if you can afford anything better.