Having used OWA in Firefox on all 3 platforms, Safari on 2, and IE6, 7, and 8, I wouldn't say OWA is crippled on other platforms. I don't really care for OWA on any of them, if you compare it strictly on usability to GMail.
However I don't see too many companies switching to GMail for their e-mail solution, given that Exchange really isn't that expensive, and is in your control, as opposed to the recent GMail outages. Exchange is like what, $750 or $800 plus CALs? IMO a small price to pay for a reliable e-mail system under your control.
On the other hand there are open source mail solutions that work very well too, however a company may incur more consulting fees than the saved on the licensing itself. It just depends.
I think the only way Google will have a chance at eroding Exchange's market share would be to make a turnkey appliance enterprise GMail system, similar to their enterprise search appliances. A web-based client attaching to a rackmount box that is still under the control of the IT dept, not dependent on Google for uptime.
It will be interesting to see what happens. I think competition is good, and the consumer benefits. However it's tough in the enterprise market, because there just isn't the backlash against MS's mail server and other products as there is on the desktop. Windows 2008 is a solid OS, and their other products are great too, with the exception of IIS, which just isn't as secure as Apache. Luckily Apache runs on Windows anyway, so no big deal.
EDIT
It looks like even the IIS thing has changed, in terms of security.
http://www.itworld.com/070907websecurity
We're still on Apache though, however it'd be nice to move to IIS given the tie-in with AD.