Need to find a good Microsoft Exchange Hosting Service. We started
with POP3 hosted by earthlink, where we had a lot of email bounceback.
We then moved to gmail's service, which has been great, but is very
brittle when multiple agents access it (our folks use outlook,
blackberry, verizon pda -- and sometimes more than one of these -- and
we tried multiple combinations of forwarding the email). We are a
small mobile organization with 5-10 accounts. Specific requirements
are:

a. The hosting company should have very good customer service and
response time. We tried the eval version of AppRiver, and have been
underwhelmed with their customer service.
b. We have large emails and don't want bouncebacks, so are looking at
atleast 2GB storage per person.
c. It should be able to handle multiple clients (outlook,
verizon/blackberry wireless)
d. Calender, Sharepoint are nice-to-have's.

Need someone to pull together options, reviews and customer
experiences to give us a recommendation.

I think 1&1 is the good choice.
But the storage doesn't fix your need.I think you can ask the 1&1 to change it.
$6.99 per month per user.
Exchange Accounts 
Microsoft Exchange Account, 1 GB mailbox space 1 account for 1 user 
Convenient Anytime Access  
Outlook Web Access   
Group Functionality and Public Folders  
ActiveSync   
Outlook Mobile Access   
Task Management   
Notes   
Journal   
Outlook Rules for E-mail Management   
MAPI over HTTP (requires Windows XP & Outlook 2003)   
Pop-up Reminder   


You can see the details
at:http://order.1and1.com/xml/order/MailExchange;jsessionid=55C8C4C2DBE6BC458083F70408217840.TC61b?__frame=_top&__lf=Static

Hope that can help you.