On my first novel, I have three betas, and I have exchanged with them sequentially. I got very, very lucky with my first beta - she is a great storyteller, and very focused on plot, story structure, pace, and characters. She read and dissected my novel as though it were her own(yet without turning it into what she would have written). Her feedback drove a major re-write that made the novel ten times better.
One other beta made it clear she had great technical skills from the get-go. She again gave me very detailed feedback, with over 200 tags in my doc, and really nit-picked the holes for me, as well as the grammar. I didn't agree with some of her opinions, but she is a very good devil's advocate. And very good with grammar. Her feedback really polished the novel.
Now, I am on my final beta. The novel is upper MG, and she is a middle school teacher. I have not asked her for grammar feedback, but rather for any remaining red flags or tweaks with dialogue. This way, I hope to end up with a well-rounded novel.
This is my first experience with beta swaps, and I am enjoying it greatly. I would say the key is to ask yourself, am I getting enough different perspectives, to address all aspects of the novel. I recommend exchanging your queries, or first two ch before committing. And be honest with yourself about whether you click. Selecting good swaps is key. I am a very detailed, thorough beta, and I wanted to be sure that I was getting the same thought and effort that I would give. It becomes obvious once you exchange a few emails if that will happen.
In the end, this is my first novel, and though I'm proud of the work on it, it may end up a "warm-up." Now, I'm writing a YA historical, but I have the added bonus of perspective about the publishing industry--which I did not have when I wrote the first book. SO I am confident it will be marketable. Now I just have to build some connections with betas who are more relative to historical -- it's a whole other genre!!