Seems expensive and doesn't offer much for the money? I was really impressed with the look ofthe HBXL software, but I don't think they do an app for the iPad? You can enter and store customer details, calculate materials, rad sizes, boilers etc and do on the spot accurate quotes using merchant's real time price lists. It'll also generate gas certs and email or send to wireless printers, generate and send invoices at the click of a button and you can set service reminders that will pop up on your computer and also send a standard email to the customer reminding them to get in touch... It looks great, but it's £200 and doesn't have an app yet. I also like the look of the plumbers CAD software they have, I believe it can import room dims from the quote software and plot out pipe runs etc giving you decent drawings to work from (great if you have an apprentice or sub working for you).
Other free/cheap apps people might like for the iPhone:
Honeywell wiring guide
Water Cylinder Calculator
Radiator Calculator
Plumbers Mate
Gas Cert UK
Gas Rate & Cost Calculator
Gas Checker