I started using the Salesforce.com/Gmail integrated product this weekend and was largely happy with the experience. The main thing for my company is getting information into one centralized system. We do so very effectively with our developer sites (wiki-based) but have been using SF.com with only middling success. My theory on the Gmail integration is that it will help to enforce information sharing amongst our staff.
My other theory is that we can eventually get rid of Outlook. If there is any app that people are more addicted to than Outlook, it's Gmail. And now we've suckered them into using it for business.
The key feature of the system is an Email-to-Salesforce email address that auto-BCC's the activity history on a contact or lead record. This is all well and good and may be idiot proof (I proved that wrong) except it's not clear that you can't attach an activity to an Opportunity directly. And while the documentation is basically clear, I am sure I am not the only moron who didn't know the deal. I filed multiple support cases and eventually figured out that I was overcomplicating everything.
The other key aspect of the Email-to-Salesforce is that you can forward previous emails to be attached to the records. They show up as Tasks and you can assign them to accounts. Very handy.
Thus far we aren't using the Google Docs or the annoying IM but we'll probably install the calendar sync as we start to go to production.
Salesforce has become a big target for SPAM and phishing and as such I don't know an easy way that they can offer this feature from systems that are not "known." Basically, Salesforce trusts the Gmail servers (I have to think there is something on the backend that makes this true) and it's own servers (including the 32 IP addresses it sends mail from).
Net result: So far, so good. It's been occasionally frustrating and I solved 2 of my own problems before I even got an email response back from the SF support team. I guess they don't work weekends in the SaaS world.
Meanwhile we're still splitting mail between Zimbra and Gmail and Zimbra SMTP can't route our domain's messages outside of the system. So, firstname.lastname@example.org doesn't go out to the internet, it just goes through the internal queue, which makes sense...unless you want it to go out to a separate SMTP server or something like Postini (which we're also test driving.)
One other note: none of this stuff is open source, but I don't know that it matters. In the Cloud at Google and SF.com we're consuming open source left and right. I haven't thought through the idea that outside developers would make the integrated application work any better.