I'm very familiar with the Universal Audio Apollo, and have been doing a
Pro Review of it over at Harmony Central. It's currently Mac-only, and while it's compatible with FireWire 400, it needs FireWire 800 to reach full bandwidth.
The Thunderbolt interface is optional-at-extra-cost; my sense is that it will ship in September, and sooner rather than later.
Windows drivers are not yet available, and I haven't gotten a firm timeline on when they're expected. When they do, I'll have to buy a FireWire 800 card for my computer...
My fave feature is that the preamps are totally colorless, so if you use some of UA's console or channel strip emulation plugs, they have a very obvious effect. Also, there's UA DSP built-in, and you can monitor through them with virtually no latency...makes vocalists very happy, and you can print with effects easily, as well as record a dry feed if you think you might want to change the settings later.
I've had very good luck using the UAD-2 Quad board with Sonar X1, so I suspect that when Apollo has drivers, it will work well with Sonar too. If not...they'll hear about it in the Pro Review.
One of the more relevant Sonar aspects regarding UA's plug-ins is that Sonar's plugs are more bread-and-butter processors, whereas UA tends to produce more specialized plugs. IOW if you want a decent reverb or delay, Sonar can give them to you. But if you want a Lexicon 224 or Echorec Tape delay emulation, that's where UA shines. They complement each other well.