I have several e-readers. The answer is, "it depends". But, the overal best usage category is using iPad with GoodReader and PDF's.
I've used tech books in the formats of .mobi .epub and .pdf. Each has their own advantages. .mobi, because the Kindle is so small and light, and easy to read on. .epub if you are an iPad user. But, the largest disadvantage of them is that those formats muck up code examples, and images (while viewable) are not easy to get to for reference.
Safari Books Online has a decent app, but it does not let you annotate or share information.
All my studies and research have been using PDF formats using GoodReader. GoodReader is more featureful than any other app out there, and you can annotate to your hearts content! Pop-up notes, highlights, arrows, etc. The downside, is that the iPad can be more heavy and bulky than the other e-readers.
But, overall, it fills the needs the best. Just my $0.02 Good Luck!