The Kindle ROI is only going to come from convenience and portability. It's not going to come from book savings.

Computer books tend to be either obsoleted or replaced by newer editions - typically with software you get an upgrade plan and always have the latest version - will you get this with the Kindle? They are also typically loaned around the office or to colleagues, you can't do that with a Kindle.

Non-computer books are cheaper to get either new or second-hand on Amazon or ebay/half.com and then simply re-sell.

You will typically recoup the difference in price with Kindle version on your resale if you decide to buy new.

The Kindle has some interesting other features like RSS subscriptions and newspaper subscriptions, etc, and while the Kindle is nice, I have no way to use my Books24x7 or Safari bookshelves (free services with my ACM membership) on it, and many of the best SQL Server or platform-agnostic SQL books are not available for the Kindle.

The Kindle is a great device, but it is not economically viable alternative to books - it's an alternative to a heavy satchel, and the value of that varies by the beast of burden. I have not yet found it worth giving it a try just for that.