In short, fantastic book, absolutely recommended to any progammer interested in learning about a fascinating and very novel progamming language. It's very well written and I was engaged from start to finish.
I think it may be of particular interest to Perl programmers because Erlang offers a great solution to a problem which is very hard to tackle in Perl - reliable, simple paralell processing. I'm definitely excited to use Erlang in the future - I've already got ideas for more projects than I could possibly find time to complete. I think its utility will only grow as processors continue to add more cores.
As a side note, I read the book as a beta PDF, which I enjoyed a lot more than I thought I would. It was very convenient to be able to cut-and-paste code from the book into a running Erlang interpreter to see it work. I got several updated versions of the book as I was reading, but the changes weren't too disruptive.
I may write a longer review, perhaps after I've written a moderate amount of Erlang so I can get a sense for what was left out. But don't wait for that - buy it now and thank me later!
-sam