For any Small or Medium business with a requirement for network servers, you can't go too far wrong with the Sun Fire range if you're on a budget.
There are two main problems that we found with these though:
1) The servers are built for use in a server room so are too noisy to be located in an office
2) The X2100 comes complete with 4x ethernet ports, but two of them won't work with a standard Linux installation as further driving installation is needed. Because of this, most customers decide to just use the two ethernet ports (Net0 and Net1)
Since installing 4 of the X2100's running Solaris 10 for a customer 6 months ago, there has not been a single failure in either the hardware or the software despite the heavy usage providing network resources for hundreds of users.
In summary, the X2100 range is great if you have a dedicated room and a small budget, but there are certainly better alternatives available.