I think I was quite an early adopter of the One Page Checkout solution from Rupostel..we wanted to try and stop losing so many people in the cart - The developer has created something great; product works really well, and even the install process is a breeze. I had some problems with my joomla install because of lots of custom scripts for virtuemart to export orders and use a UK payment processor, we had some problems with order status (our script requires al orders are set to pending) ... Stan worked with me over at least 4 nights on skype to fix every single problem (90% of which were because of our tricky joomla setup) and never even asked for the install fee (so I donated to him because the product, and help was unbeatable). He even sent me the .css file he was using for the checkout, to save me time creating a new one. If your looking to make your checkout process faster and slicker, look no further than this great piece of code. Any problems you have, Stan will fix within a day or two I'm sure, but I think the software is almost completely bug free now since I was installing it a few months ago...a very strong script, and I will be installing it on other domains as I build them, as it's the final piece to the joomla e-commerce puzzle :) It even allows for a "guest checkout" option, which is really useful for those 1 time sales websites where people dont want to make an account.
Thanks Stan...you made my life easier :)