I have a 7" Kindle and "comfortable" is not the best way to describe the experience.
- Always on landscape mode
- Some books will have large fonts, giving you a "claustrophobic" sensation. Maybe I'm overreacting but sometimes it does
- Books with small letters: You'll find yourself zooming in sometimes.
There are some readers that improve the experience, but, all in all, if you have the money. Get a large screen tablet. I'd say a 4:3 screen will give you a better reading space.
Also, there are softwares to convert them. The end result is not good. The Kindle version (From Amazon) also doesn't look 'right' for programming books. Sometimes you get confused with text vs. code block. Images and tables will break.
I have a lot of programming books and despite all that I rather read them as PDF. It's not the perfect solution, but, it's all I have.
EDIT: I use a mod for my Kindle, I tested a lot of readers.