I've been a MM user since day 1. My usual advice to people is "its a good deal if you are really going to use it."
By this I mean "if you're just wanting an IMAP email account, or JUST the syncing, then its not worth it." Even though its easy to find MM packages at less than full retail, it's still money, even if its only $5-8/month.
BUT if you think you will take advantages of a wider spectrum of the services offered, then MM becomes a VERY good deal. Beautiful photo galleries, lovely personal websites (with no hassles about "bandwidth"), tracking a lost or stolen iPhone, 20GB of iDisk space (that's 10x what dropbox offers), **AD-FREE, SNOOP-FREE** email address that's IMAP and easy to remember, iPhone push syncing, and more ... for between $5-8 a month depending on where you buy it? That's a no-brainer in my view ...