Users in this forum run almost everything server-related on SME. From campus-wide servers with +500 Users to small home-servers with 2 users, a little filesharing and web-access.
SME will do all that, if you do not use flakey hardware. Web-poll here some time ago showed, that about 75 % of the installations are in a SOHO < 25 user environment. And that is, what SME was made for.
I use it in office as AllInOne for 8 users (with static IP, ADSL and quite heavy mail-traffic) and at home for family use (7b8 version, 4 user, little traffic).
On office-server we are running a german TAPI software AGFEO TK-SUITE on top of SME also. (Web/SQL software with COM-port interface to the PBX). It was quite hard to configure that, but it worked ever since.
I'm running e-smith/SME for years now and without any troubles, compared to MS NT, w2k and such.
The only point is: Do not change or "mod" your SME-installation too much. This will cause heavy problems, esp. with "roll-back" or upgrades.
And if you want to do very special tasks (a standalone faxserver, a customized router, advanced windoze fileserving or such) it could be useful to install another distro for that and leave out the whole SME template system, because it can be hard to configure the functions, that are not implemented in server-manager.