We used Harmony Valley for the first time last year, and will definitely do it again this year.
An aspect of them which I really like is the concentration on heirloom varieties. So you end up with a lot of things that may be difficult to impossible to find elsewhere. This could be a possible downside for those who don't like to experiment with new things. But we really enjoyed it.
I am not familiar with Foxtail, but I have had a Harmony Valley CSA for about 5 years now. My husband and I get the every other week veggie and the full fruit share. The only complaints we have is, with the veggies, the end of the season can get a little monotonous (only so much radish you can eat!) and, with the fruit, sometimes our entire box will be ripe RIGHT AWAY and there is no possible way to eat it all. We tried their cheese share, but found it to be too much of the same...lots of cheddar, not so much exciting stuff.