I've done my share of railing against Apple for MobileMe so I think i can give as much of an objective opinion on this as any. I use both services as well (Gmail and MobileMe).
Gmail is free so if cost is an issue, it's a non-issue. MobileMe gives you 60 days to try it out for free. Some on here like it well enough (some love it) and others (like me) have issues with it. I'm not going to crab it on cost because of two things....
a) It can be had for around $60-$70yr if you buy a retail kit from Amazon.
b) The (combined) features of MobileMe priced together as part of the package is a steal.
E-Mail: Gmail is free, and offers free iDevice syncing (ala ActiveSync). MobileMe is Mac-focused e-mail, no ads, and is a more traditional form of IMAP service than Gmail. Most people (incl. me) have both a MobileMe and a Gmail account--it costs nothing to try them both and see which one you fancy better. Most people I know use their @me.com account as a primary/important address, and their Gmail address as an archive and mailing list address.
Calendar: Google Calendar is free. MobileMe calendar is fancy. I won't delve into specifics but the new MobileMe calendar is quite nice. Google Calendar is a great system, very utilitarian--it just works. MobileMe Calendar is fancy and follows the Apple tradition of "simplicity." Try each, see which one you fancy better.
Gallery: Google has Picas, not part of the Gmail system but still a Google product. It's free up to 1GB and an additional 20GB can be had for $5/yr. MobileMe Gallery lets you share video and photos. Of the two, I do like the Gallery better than Picasa for sharing photos with family/friends/etc. Picasa is great as a freebie for screenshots and eBay photos. I wouldn't use MobileMe gallery for things like that. Picasa is great if you prefer a web-only system, but Flickr is a lot more simplistic. To each their own but I give MobileMe the nod on photos and video.
Find My iPhone/iPad/iPod: This is now free for iP4, iPad, and current gen Touch owners. I've had this for over two years and it is VERY handy. If you have an older device, it requires you to use a paid account. If you don't have an iDevice, then you can skip it. I will say this--there are some people who buy MobileMe just because of this feature--and only this feature--but I think that's silly. There are some crabbing Apple bc they are now giving away free for what they paid extra for up to a few months ago--I still think it's stupid just to pay for MobileMe to get Find my iPhone but to each their own.
Back to my Mac: My wife uses this a lot. Think "Go to my PC" but a lot more simplistic and no monthly fee. I only use this service sparingly because most of my stuff is in cloud storage (iDisk, Dropbox, or otherwise) and don't need to access my home iMac very often. This is one of the services MobileMe has that I can honestly say there is no equal for the price unless you set up your own VNC server and client. It's very easy to use.
Contacts: Simple syncing to the cloud, your other Macs/PCs, and iDevices. Nothing fancy except it supports Groups whereas Gmail's incarnation doesn't. It also supports the full array of Contact Fields in each contact entry--Gmail's doesn't, unfortunately.
iWeb/Domain Forwarding: Strangely enough, I am starting to use my iWeb to host some school-related presentations. I own my own domain name and have it forward to my iWeb site--it works really, really well. iWeb is still the unsung hero of the iLife suite if you want to get a nice, fancy site up in under an hour. As far as MobileMe is concerned, it works well for hosting the site but isn't necessary--iWeb will let you upload to an FTP server. I don't own server space so I just use MobileMe to host, it works very well for this.
Storage: Gmail doesn't offer a tradtional "cloud storage" service like iDisk or Dropbox. Google Docs is about the closest thing for document storage you'll find in Google's offerings. iDisk is sort of a red herring for some, and Dropbox has been a favorite on these forums for a long time. I use both Dropbox and iDisk. iDisk storage is shared with E-Mail storage up to 20GB (upgrades can be purchased as high as 60GB). Dropbox is free up to 2GB (I have over 14GB from .edu referrals, so I'm a happy guy) and offers some "goodies" iDisk doesn't (30 day retrieval of deleted items, secure storage, etc.).
Apple has been arguably tone-deaf to iDisk when it comes to features and security, but where it does shine is simplicity. iDisk is a lot more simplistic than Dropbox, it's native to the Finder, and is a tad faster for uploads and downloads than Dropbox is. As far as security, iDisk is now SSL secure for uploads and downloads (Snow Leopard only) whereas Dropbox is also secure for storage on the server. Why? Dropbox rents storage from Amazon S3, Apple owns all their own servers and storage. I'm glad they implemented SSL for data transfers, so it is debateable as to whether one is more or less secure than the other considering the circumstances each implements it. Presumeably, the lack of SSL for Leopard (10.5) and later is due to a Finder limitation....that's a guess, anyway. Snow Leopard didn't have SSL for iDisk until recently (10.6.5).
What does this mean? Authentication (username/password) was always SSL secure but the transmission of data wasn't. With SSL, all your transfers (not just authentication) are encrypted while in transfer.
Offline syncronization is another factor: Dropbox uses a proprietary application aptly named "Dropbox"and iDisk uses SQL-like synchronization for offline data with the Cloud. Dropbox's biggest compliment has to be how compatible it is with everything, such as editing a Word or Excel document and having it upload each change as you save it. iDisk does everything in one big swoop--sort of a giant sync en masse. For me, I found Dropbox's implementation was the most compatible but also used the most resources, and wasn't friendly to laptop batteries. iDisk will not sync if you are on a laptop and are low on battery--it polls an error in the Console "File Sync" log in the vernacular, something like "not a good time right now." Very cute. It also doesn't eat CPU or RAM like the Dropbox app does. For a desktop, it's not a big deal but if you're on a laptop, it can make or break it.
As far as iDevice apps, each are free so you can try them for yourself. I like the Dropbox iPad app very much but viewing PDF files on it is arguably slower to resize and open than using the MobileMe iDisk app.
Sync: Really simple. All your Calendars, Contacts, Bookmarks, and E-Mail is "pushed" to your other devices, rather than having to sync at intervals. It had a lot of issues when it first came out (July '08 anyone?) but reliability for syncing has improved a great deal. Some people love this and use mobileme for nothing else than syncing their Macs and iDevices. I love the fact that Bookmarks sync between all my Macs and iDevices, and are pushed almost instantly. It's one of those things you never thought you'd use but will appreciate quickly.
That's all, I hope you try both and see which one works best for you. Best of all, you have options, and quite a few of them. I think it's best to start with MobileMe since it's got a long 60 day trial and you can compare it to other offerings, see which one fits you better. It's also the most simplistic of the group, and that might be the best thing about it.