I've done research myself and the Sigma units support ADI and TTL for Sony. It also supports wireless and high speed shutter synchronization, but not at the same time like Sony models.
From what I've seen, it looks like a very capable flash unit. It is a very good alternative, but for a little bit more, you could get the EF-530 DG Super model, which is slightly more powerful. It also is 11 guide units (meters) more powerful than the 42 and still supports just about all of those functions.
If you want an honest opinion from me, I'd go for studio strobes, which is what I'm actually trying to do. Alien Bee's flash units are extremely powerful and versatile. But I don't think you're ready for those yet.
The EF-530 DG Super from Adorama is $269 USD which is $30 cheaper than the HVL-F42AM.