I can recommend Zimbra Open Source Edition, as I installed it instead of MS Exchange at two small companies. It'll integrate with Active Directory, if you want it to, or it can host its own directory information. The OS edition is nice - installation is straightforward and easy and the software performs well enough. You'll have to come up with your own backup solution for the OSE, though there are a number of sample scripts floating around.
I don't know about the value of the subscriptions for Zimbra. It will depend on how many users you're licensing, and whether you need to factor in MS Office into the equation. The 'Professional' edition of Zimbra, which includes an Outlook connector runs $875 a year, the 'standard' edition, which doesn't include the connector but does have backup integrated in, is somewhat cheaper - $625 per year.
The web client is definitely very nice, and it can be downloaded as a standalone package as well, to interface with other mail services.
PoS I have no direct experience with, unfortunately - google turns up stuff on Openbravo POS, among others. I know nothing about these products, though. Check out Zimbra, if you're serious. I've used it, and can definitely recommend it highly.