Although there is a lot to comment on about this program--including some very redeeming features--I am going to concentrate only on a feminist perspective, as others have mostly covered the other qualities.
I have not watched all of the episodes, but I would like to say that the episodes I have watched, I have some problems with.
I am glad that we have a show depicting a large amount of girls participating in an active and competitive sport. I am unhappy with how those girls are depicted, especially when compared to the boys.
First of all, as several others have said, I do not like how incredibly and unrealistically skinny all of the female characters are, especially considering that these characters are between the ages of ten and...fifteen?...and should not by any means have shed all of their baby fat yet (although I am very grateful that they did not decide to exaggerate anything in the chest area), especially when the boys seem to have a far more proportionate builds. Furthermore, all leadership roles seem to be filled male characters--among horses as well as among the humans. The dog appears to be the wise mentor of the two female animals (a pig and a cat), the boys at the horse farm always seem to be guiding and giving advice and compliments to the girl riders but not vice versa; the wild horses that appeared in one episode were, of course, led by a stallion. When a female character takes the lead, it is usually only with other females or it is done with catty motives.
Furthermore, I have yet to see a single argument escalate between two male characters--again, I remind you that I have not seen the entire show--and yet, in nearly every episode, we have insults flying between the girls or otherwise haughty and stereotypical 'catty' attitudes between them.
I have to include, however, that this IS a show that depicts females being athletic, which is very, very important. For that, I commend it.
The show does have many good features; I would not, however, count most female-empowering themes among them.