I use a Treo so I haven't had any issues with SIP. I did notice that my battery drained a little faster but I was texting like crazy with this. I love the look of it and the usability. I hate that you can't customize it. I have become a tweaking junkie. If it can't be tweaked, I try not to use it. But other than that, pretty cool. I'll keep it for a minute. For those of you who don't like it, use PocketCM. You could use that as your SMS, + it's threaded. If you want it check out this site from the developer (and it can be tweaked and skinned)
http://www.pocketcm.com/contact.php. + its freeware and free is always good, ya dig