I have been using gmail for quite a long number of years and decided last year to use 'Google Apps for bussiness' to test my work email in google's environment and it is really great. Here are just only some of the reasons:

Automatic email filtering

I use email almost 24 hours a day and checking it on mobile phone was becoming a hastle since all my filters were set up on desktops email client ( Thunderbird ). Now some emails just skip Inbox ..

Search

Do I really have to compare Thunderbirds search engine with googles ? :)

Spam

Alhough I don't recieve much spam gmail is really the best tool to fight it.

Filtering email: Folders vs tags

In Thunderbird you can move email from Inbox into one filter-like-folder while google uses tagging which means that email can hold multiple filters instead of just one.

If you already have or want to have email on your own domain but google hosted for free click http://www.google.com/enterprise/apps/business/pricing.html .. It takes around 30 minutes to setup everything.

P.S. There are some downsides to this ... google will have all your email :)