Here are my thoughts on the Sonus line. When I got started a few months ago, I pretty much ordered the entire Sonus catalog. Now that I've had the chance to use the products, and a chance to use a comparable product, I'll give you my dos pesos.
SFX-1 - It's supposed to be their most aggressive polish...Isn't very aggressive at all. Switched over to SIP and IP. The difference is night and day.
SFX-2 & SFX-3 - These two are pretty good, especially the SFX-3. SFX-3 is a nice finishing polish, but pricey.
SFX Pads - I don't really like them at all. Switched over to LC CCS pads....HUGE improvement.
Sonus Towels - Like the OP mentioned, these are great. No complaints at all...especially with the Sonus WW.
Sonus Wash Mitts - No complaints here. Although, when it's time to reorder, I'll probably try something different.
Sonus Interior Products: Total Eclipse, Cockpit Detailer, Leather/Vinyl Cleaner and their leather conditioner are all solid products. Again, they're a bit on the expensive side.
Sonus Rim Bright - A very mild wheel cleaner. I prefer P21S Wheel Cleaner.
Sonus Ultra-Fine Clay - Personally, I like it. BUT, I've only had experience with two different brands of clay. Sonus and the generic Megs clay (JUNK).
Sonus Bumper/Tire Gel Dressing - Good stuff, especially when used over the Total Eclipse on tires.
Sonus Der Wunder Wasche - No complaints, but again, this is the only speciality MF wash I've used.
Sonus Gloss Shampoo - I'm not that impressed with this. It seems to have a decent amount of lubricity, but the suds just don't seem to last in my bucket.
I hope this helps. Overall, I think their products are ok...some better than others. Also, they are a bit overpriced.