The downside is that users aren't connected to each other - you can't merge employee schedules and tasks can only be set by the user. Introducing Web E-mail. But if you're short on cash, it's a good way to get started. By outsourcing your e-mail, you're at the mercy of your ISP's server. If you're having e-mail problems, you want a real person to talk to.
Some clients provide extensive, free support via telephone, Internet and e-mail.
Also check the quality of the clients' online tutorials. While e-mail programs are pretty straightforward, it can be hard to find some of the more obscure but useful features, especially in sophisticated programs. These could be deciding factors if you're on the fence about two products. You've chosen in-house groupware. This is the highest level of e-mail capability available currently.