I've tried a few over the years, and I've yet to find one that can actually deliver on a service level agreement, would you mind sharing which companies you have good experiences with?
I'll have a look back for the main one I used and had success with. They offered delivery reports, raw data sending etc I never had any trouble with SLA when I was using them for sending out ringtones. Will see if I can find the provider I used. It would have been British.
I work with a mobile marketing company that uses OpenMarket as their SMS hub, and I have been quite pleased with them so far. They provide a consistent API and callback feature for sms that bridges over to the mobile networks.
I used one such service from 2000 or so when I ran a ringtone website.
It's a solved problem. Just search Google for "bulk sms gateway" etc and you'll find literally hundreds of providers with various APIs and so on.