My firm has been using Function Point for over 5 years. Before we decided to go with FP, we created an in-house team to evaluate FP along with 5 or so other applications. FP was the clear winner and we have not had any regrets with our decision to go with FP.
Some of the other applications we reviewed offered built in accounting and we really did not like this approach. We felt like it was better to let the Quickbooks of the world worry about the accounting as they have hundreds of experts dedicated to keeping their application updated. We wanted our solution to focus on all the other parts that are necessary to running a profitable agency and dedicate their team to making that the best that it could be. FP's seamless integration with Quickbooks, gave us the best of both worlds.
FP listens. If we have product improvement suggestions they listen and often incorporate those into their ongoing software updates. Not to say they do every crazy thing we suggest, but the stuff that makes sense for all their clients always make it into the updates.
Finally, A company is only as strong as its support and FP has delivered to our firm the highest level of support. We deal with other online software solutions on behalf of our clients and no other company comes close to the support FP give us.