This review is from the perspective of a caretaker. I can't speak to the employer experience.
Not User Friendly: Care.com can be quite aggravating because any changes made to one's profile require a "review" by someone with the website before it actually appears on your public profile. The review doesn't seem to involve anything more than an annoying delay before changes are posted. Also, if you want to get any real functionality out of the site (like, the ability to message families) you have to pay extra.
Poor Quality of Interactions: I will say I did work for one woman from Irvine, CA on two separate occasions and she was very nice and she and her family tipped me generously, and another family gave me a very professional declination to an application I sent, so not all my experiences were negative. Overall, though, employer-users seem to have little respect for the caretakers on here. One woman, an 'interior designer' from Orange, CA was rather rude at various points during the process. Besides actually withdrawing her offer of employment several hours after asking if I could start the next day, she followed that BS-semi-nice text with a curt "Please respond that you received this." Like many employers I think she assumed I would come at her beck and call, which obviously wasn't going to happen since I am also attending graduate school at the moment. I stood by the hours she initially offered me, which I think pissed her off. Also, when we met for the interview and I mentioned that I pay taxes on my jobs, she immediately slipped into a snarky tone, saying, "Oh, well, I'll give you until Monday to decide whether this is really something you want to do..." People do understand that "under the table" = illegal, right? If I had worked for her I would have netted about 500-600/month--how to you explain where that money went when tax time comes? A "professional" should know better.
Another woman messaged me recently with another rude message that didn't touch upon scheduling, pay, etc. Again, an assumption that caretaker-users on Care are so desperate for work they'll do anything an employer wants. Um, no.
I also got at least one short, ambiguous message from a man asking if I could work that day. No other information. Hmm, I wonder what he wanted me to do? Especially after I was informed by Care.com later that his profile had been deleted by administrators--even though they wouldn't explain why.
I'm sure the site itself would argue they are not responsible for "some bad users," but in a way they are, at least to some extent (granted, some of it is due to the generally unregulated business of assisting/caretaking, but still Care.com is not helping the situation). Namely, the informal nature of the site invites informal communication and hiring practices. Caretaker-users should be able to rate employers in the same way employers can rate/leave a comment about us. At the very least there should be a more open means of reporting questionable people. And Care.com needs to enforce the payment of taxes.
Cost: While not receiving any questionable charges, I thought the prices charged for aspects that arguably should be free (like the messaging) was way too much, especially in light of the poor quantity/quality of work one is likely to get through the service.