Security and speed. The core for imperva is very impressive. The stuff that is all in the background runs well and stumps security professionals. We actually give them vpn access to test us from internal due to how well imperva stumps them. This allows us to stay up to standards in case we need to change services for any reason (though we wouldnt want to).
I was kind of annoyed about the nearly complete API. I was unable to get something that was available to resellers only which did not seem to be reasonable.
Imperva is very impressive for its core feature set. Imperva has made security analysts scratch their heads and have us allow them in from inside so they can actually hit something worth while. We are very confident in the reports we get from imperva and its bot identification has allowed us to plan for bandwidth appropriately. Identification for good bots (people who hit our site using automation, but for good business reasons) has allowed us to work with our customers who use our services in new ways.
We have solved:
Increased Security, External Cache/Proxy, Simplified management including certificates, load balancing.