I find .Mac to be worth every penny for me.
The most important reason being, if you have multiple computers (desktop & laptop), then you can keep all of them synced together with the same information.
The level of integration between applications, the OS, and .Mac is certainly unmatched.
I also have multiple email accounts - Yahoo, Gmail, AOL, and others - and they all seem to bring in lots of junk mail. This is my theory:
Since all of those email providers are "free" to use, they sell your information to advertisers and other companies, which results in excessive spam.
I remember the day I signed up for Yahoo mail (the absolute worst). A couple of hours after signing up, I was bombarded with junk email! How did these spammers get my email address so fast? Don't be stupid! It is because Yahoo (and other "free" providers) sell your information. Do you really think that everything is free and that Yahoo does not benefit themselves in any way by giving you a "free" email address?
It is like the "free" music and television show trials going on right now. Sure, you can download shows and music content for free, as long as you are also forced to watch or listen to the advertisements that come with it.
With .Mac, I never worry about spam (maybe received one or two spam emails in the 4 years that I have had it) and I also don't have to look at advertisements all over the same page as my emails.
I will admit that Google is certainly the best among the other alternatives (Yahoo, AOL, etc.). I have almost no spam with Google as well. I also really admire the company.
So, I guess to finally answer your question, you may only benefit from .Mac if you have more than one Mac computer and you want a more transparent integration between the Mac applications and the computers - iCal, Address Book, iPhoto, iWeb, Safari, etc.
Oh, and I forgot, .Mac gives you 10GB of network storage for backups or file transfers. Does Google do this? As far as I know, the "free" storage Google provides is used only for emails. Can anyone confirm this? I don't want to post misleading information.