I'm not easily sold on conferences. When I worked at Indiana University, our bosses offered us as much travel/training budget as they could, in an attempt to make up for the lousy wages -- so, needless to say, I've been to a number of them, mainly as an attendee, occasionally as a presenter.
Of all the conferences I've attended, the only one I can recommend without reservation is the Microsoft Professional Developers Conference (PDC). For anyone working in the MS development ecosystem, I genuinely think the PDC is a worthwhile investment.
There's a lot to get out of the conference:
-- When I was a dev writing for the desktop and server, there were always more sessions that interested me, and had direct impact on my job, than I could possibly attend
-- Real MS devs, PMs, and MVPs working on the products you code with, and against, every day are there, eager to help you with your problems
-- You can network with other professionals in your field, including the leading movers and shakers. The Open Source community is also well represented... back in 2003, Miguel de Icaza gave me a Mono t-shirt!
-- Lots of vendors are there to talk to you about their products, and to give away cool swag
-- MS also tends to give previews of exciting stuff -- this year, some Windows 7 bits will likely be the highlight
I won't be at PDC 2008 this year; in order to free the maximum number of slots for customers, we limit the number of MS attendees. However, Windows Mobile and Embedded content publishing will be represented... more on that as we get closer to the PDC!
If you can make it, make sure you register soon -- the conference is almost full!
[Edit: Constanze's got more details about the Windows Mobile offerings at PDC 2008]