Tempted to switch to an Interchange Plus account anything I need to know?

I've been processing with Merchant Warehouse for a few years now, processing around 6k/mo selling low risk physically delivered products, when I applied I was getting just about the lowest rates around, however I recently was quoted a price using interchange plus rates which seems much more competitive than the traditional tiered pricing. I just wanted to know if anyone was familiar with some hidden fees I should expect, or any other reason not to go with this system. The name of the company is Retriever Payment Systems, if anyone has used them some feedback would be great.