My experience has been pretty similar to others' here. But since I like to hear myself talk

I'll repeat what's already been said.
I'm on my second ebook reader -- slipped my first in a back pocket, then sat on it; if only it'd been a paperback -- and I'd have to agree whether I've saved money depends on how you count. Prior to picking up my first reader, I used to hit the local bookstore every 4-6 weeks, dropping about $100 per trip. With my reader, and all the free content available, I haven't done that for myself more than once in the last year, so I could say I've saved $600 (six trips @ $100 per). (That's not to say I've quit buying pbooks; with a daughter just reaching reading age, however, it all gets spent on her now, rather than me. The price of children's books these days is
unbelievable.)
Conversely, I could say I'm saving $8-10 per book with my ereader. However, that's a bit apples-to-oranges, as I now read almost exclusively classics and PD -- stuff I never bought when I was collecting pbooks.
Even at $10 saved per ebook, however, I'm still in the negatives. My first reader having bit the dust before it'd paid for itself, my new reader has a larger hole to dig me out of. I guestimate I'll have to read another 30 to 40 ebooks to break even.
As mentioned, the biggest change with my reader has been in what I'm reading. There's both good and bad there. I definitely miss my Harry Turtledove fix. On the other hand, I've discovered Raymond Chandler (through MobileRead); not being a mystery aficionado, I never would have picked him up in pbook format.