I have to respectfully disagree with strongm, we do a lot SBS implementations and aside from the limits on the number of users and the fact that SBS needs to be the FSMO role holder there is little difference. In fact you can actually gain some functionality such as the Remote Web Workplace (RWW), a feature which has been so successful that it is being ported over to a Medium IT Server.
There is no difference in the SBS Exchange from any other Exchange Standard server.
Contrary to popular misconception, you can add additional servers to an SBS network and you can also make them DCs and GCs, just not FSMO role holders or DHCP servers.
As ashleym has pointed out you can purchase the transition pack if you need to step up beyond the 75 user limit. The cost is roughly the same as if you were to purchase the seperate products for Exchange, SQL & ISA and you need to purchase new CALs. Where you save is on the fact that your AD remains intact as does all of your data etc. Once the transition pack has been loaded you retain all the functionality of SBS such as RWW but you no longer have the 75 user limit and you can break your products such as Exchange off to other servers. (Note: you must order seperate media for this, the transition pack will let you use the same keys as from SBS)
Bottom line for making a dicision to go with SBS or not should be:
1. Max number of users you need to support
2. Will you need more than one information store
3. Budget
Additionally evaluate if you need either ISA or SQL as they are only included in the SBS Premium Edition. We tend to use the need for SQL as the litmus test for our customers since ISA can be replaced by an inexpensive firewall like a linksys for most small businesses who won't necessarily miss the application level filtering that ISA can do.
I hope you find this post helpful.
Regards,
Mark