there's a 60 free trial so you can test drive it to see if it's worth it to you.
i have it, and i bought my sub via amazon for only $35. wherever you choose to purchase your sub, with it you not only get the IMAP+idle email which everyone seems to focus on, but you get 20GB of idisk space that can be split up with your email storage however you choose. you also get 200GB of bandwidth per month. compared to other online storage, realize that even at full price, it's about the same price or cheaper than most other popular equivalent solutions.
you also get an online photo album, email synced across all devices and webmail access, ical syncing, notes, to-do list syncing, address book sync, safari bookmark syncing, and data backups (with software). lastly, for those who work from multiple macs, you also get "back to my mac".
i use my mobileme account to sync my contacts, calendar, and email specifically for my iphone but also use the idisk backup, public folder sharing for iphone videos, host online photo albums that i manage with iphoto and upload from my iphone camera, mirror my macbook's download and documents folders in idisk. all of this is also accessible on my windows home server/desktop with mail and calendar accessed with outlook and idisk mounted as a netshare drive. i also use mobilefiles on my iphone to access my idisk from my iphone. the killer feature is being able to "print to pdf" any documents directly to my mirrored idisk documents folder then having them accessible thru the mobilefiles app on my iphone... receipts, purchase orders, etc.
in the end, if you managed to read all of that, i have found mobileme to be worth the $35 i paid for it, and if unable to find a similar deal, i would easily pay $100 a year for renewal. it suits my needs and even solved a few issues i had no idea it would be able to.
the ONLY problem i had with it was setting up ical sync to my new macbook... which was promptly handled within 8 hours by the mobileme support team by rebuilding my ical sync data.