Really just depends on the material. I've bought ebooks for single effects and for compilations. If an effect is significant enough to stand on its own merits, fine. Bill Montana's Pulse is one example that comes to mind. If it's not stand-alone material, then it simply shouldn't be released as such. Mick Ayres' Acts series comes to mind - he's releasing his entire act, so to release any effect solo would be pointless in my opinion. OF course, if I'm not interested in the material, then I wouldn't even bother downloading a free copy.
I should add that being someone who lives in Australia, ebooks have the obvious advantage of having no shipping and being far more instantaneous.