instead of Microsoft Exchange.

Any warnings?

*cricket sounds*

It came up in my search as being very popular, but no direct experience.

We use it where I work.  It's okay, definitely better than my experience with Exchange (which was that anybody saying they didn't have problems with Exchange was blissfully ignorant).  You might be better off sticking a UNIX server in there though.  The email solutions are more advanced and more reliable.  One of our problems with IPSwitch is that it will occasionally lock up and delay processing on mail for several days.  Awkward when some of those mails are to a fire damage mitigation company, dispatching a notice that somebody needs their burnt out house boarded up.

If you don't know UNIX very well though, you'd need to hire an outside firm to do this.  There's probably one in your area though, and if you happen to be in the Midwest I can even recommend one.