Just an FYI - I updated to Gravity Forms 1.5 the other day with no problem. I've got pretty straight-forward forms on my sites.
Yesterday, I installed the Lite version of Cart66 onto one of my sites just to check it out prior to purchase. As it turns out, Cart66 does not work with Gravity Forms 1.5. After a visitor submits a regular Gravity form, the e-mail does get sent along, but the visitor gets the following error:
Fatal error: Class 'Cart66GravityReader' not found in /home/radiovic/public_html/wp-content/plugins/cart66-lite/models/Cart66ShortcodeManager.php on line 461
This has resulted in a dozen visitors sending duplicate e-mails to us through the form since they figured the send failed.
Let me make this clear, all I did was install and activate Cart66 and create one product. I DID NOT do anything to create a form in Gravity tied to Cart66.
There is no support at all for Cart66 Lite - and I can not find an active user community out there. I asked Cart66 if there was an active user community for the Lite version other than the WordPress plug in page, and did not get an answer. They did tell me however "just so you know, Cart66 does not work with Gravity Forms 1.5. This may be the issue".
So if you're running Cart66 and a previous version of Gravity, don't upgrade Gravity to 1.5.