One problem I'm having with it at the moment is that I've got a large mailing list (almost 11,000 subscribers) and I'm currently having to mess around with the config file to set it to send out mails in batches and / or throttle requests to the server because it keeps exceeding the 'max questions' that my host will allow. This is a common problem with any php mailing list as far as I can tell but there's more and more talk about solutions on their support forum so I guess it's only a matter of time before you can configure throttling and batching through the admin section.
I'd love to see this ported to XOOPS but I'm a total novice with PHP so I can only hope for some other nice techie in here to grant my wish! The only reason I can mess with the config file in PHPlist is because there are examples of hacks on their support forums...