While the module works fine on its own BEWARE that if you are forced to also use the official WHMCS paypal module for existing customers that don't want to migrate over you will have MANY MANY bugs.
Contacting support will result in responses that are weeks apart, and if you purchase the "fast track support" they will not have 1hr response times as they advertise. Additionally any fixes they upload to your site will be untested and they will be testing those fixes out on your live installation. We now have literally hundreds and hundreds of transactions that are blank and not logged in any way due to these untested changes they uploaded to "Fix" the issue.
If you contact their sales they will respond very very quickly, but that is because you are interested in getting service and paying them more money. Once you're a customer it is minimal effort support with no interest in solving bugs with their software.
Summary: While the module by itself is great beware their support. Contacting them will be a very slow process and they will cause far more damage than good in troubleshooting the problem with each response that comes about once a week or so.
Edit: Surprise surprise as soon as we posted this review they responded to our ticket 28 minutes later mentioning the review. Shows that they are indeed ignoring tickets instead of solving them.
Edit2: In response to this developers comment we need to clarify some things since they aren't being honest. What this developer doesn't disclose is that for nearly a month we repeatedly asked them to revert the untested changes they did to our production WHMCS. They continually said there was nothing wrong with their changes and that WHMCS was flawed. After a MONTH of them refusing to go in and fix the problems they caused we decided to take matters into our own hands by uploading a fresh copy of the module removing all of their poor coding that they added. This solved all the problems they introduced and have said below are caused by other things. Sadly the original bug that has been there for nearly a year is still present. There is nothing wrong with our WHMCS and the WHMCS staff have confirmed this. It is simply the MyWorks Design poorly designed and doesn't work well with the official PayPal module. As for their fasttrack support they "guaranteed 1hr response times the life of the ticket" it is unfortunate that expecting 1 hour response times is an "unreasonable attitude" as they have put it.
Again please be wary of this developer. The module works great on installations where you NEVER use the original WHMCS Paypal such as our second license but if you have to use both at the same time be prepared for many bugs that the developers will not fix and will refused to acknowledge when they cause additional problems in your production environment. To avoid this don't give them access to your system. Upload any changes they want to make manually and then test yourself so that you can remove such changes when they cause other bugs.
We're sorry to hear that you're having issues with our module. Obviously there are two sides to this story, and we won't be disclosing the content of your support tickets with us to clarify our position.
Unfortunately, as we mentioned many times in our ticket with you, the errors you're encountering are purely localized to your WHMCS installation. We showed you specific screenshots of very visible WHMCS discrepancies in your system, yet you chose to repeatedly ignore them and demand we resolve an issue we had no control over. As if this couldn't be any clearer, the second license of our PayPal Billing agreement module you have with us was showing none of the same issues, and we repeatedly showed you examples from our live and staging test sites that proved this was not a module-related issue.
We have over 1000+ happy WHMCS users actively using this module (as evidenced by hundreds of reviews and feedback across the WHMCS network), and not one has reported the same issues you asked us to assist you in resolving. We repeatedly attempted to clarify this with you, however you chose to ignore us.
When you didn't receive immediate replies to your fast track support ticket, we refunded your $24.99 payment after you expressed your extreme displeasure in not receiving hourly updates. Your unreasonable attitude has assisted us in better clarifying our fast track support policy for future clients, we appreciate your feedback.
In summary, we advise you contact WHMCS to troubleshoot WHMCS-related issues, set up a staging site and attempt to reproduce the issues you're seeing on your live site there. The second license of our module that's working perfectly fine for you is a great example of the reason this should be done.
This module is no longer being developed. Updates are released less than once a year and only if you open a ticket about it. Additionally they now take
more than a week to respond to tickets and do not check their work before releasing.
Avoid this addon and use some of the other alternatives.
EDIT: Also, Seeing as Bob below is willing to share your private communications with the entire world more than proves you should stay far away from circle tree. They have no regard for the privacy of your account or information.