Nice! I primarily use http://billable.me/, but I think this is going to be work better.
Nice concept, and it looks alot leaner than alot of the existing solutions so you could be on a winner! One note there are a few spelling and grammar problems in the default copy, don't know if you're the developer but if you are might be worth having a look.
I agree with your suggestion. Actually, i am not the developer of this. I found it useful to me so thought to share with my fellow designers.
I am exactly using https://www.myinvoicegenerator.com for 3 Months. Its One-Page and gives the posilibty to download my invoice as PDF. Good one for an online invoice generator. Recommended!