Michelle,
We've used Sonlight for 3 1/2 years (Core P 4/5, K, 1, and 2). This year we will be finishing up Core 2, revisiting Core K, and adding in Core P 3/4. We love it for all of the reasons already mentioned: mission focused, literature- based, ability to combine children, etc. I also love that they have everything already scheduled out for you including some supplementals. But, I especially love that I can reuse a core with my other children which saves me LOTS of $$$ in the long run.
SL does offer their own Language Arts; we've added this in too. I'm on the fence as to whether I REALLY like it. Unlike traditional LA, it introduces a new concept and then practices it every so often. Having been taught "old school" the more relaxed approach to grammar in the early grades bothered me a bit so I've chosen to supplement in this area over the years.
For our family, SL has been great. My children love "Box Day" when all of their new books arrive and I love that they are excited about learning! SL has been a wonderful company to work with...their customer service is exceptional!
If you have any other questions, please feel free to email me.
Blessings!
A.