For a newbie owner they are a great service.
I am happy with them. When I was a new owner, especially, it was a big help to have their guidance in picking a shop and deciding which "discrepancies" to decline, when the shop tells you what they found. And you learn a lot from this interaction, so that you will eventually not need it.
What they are: essentially a middleman, acting as an intermediary between customer and shop. This sometimes frustrates a shop, but it works. And they have good people, typically somebody who is (or was) a director of maintenance at a big shop.
What they are not: They don't repair your plane or examine it. Nor do they study your logbooks regularly or keep track of when it is time for you to get something serviced (those things are still the owner's job).
They just recommend shops and then communicate with you and the shop using posts on tickets, which are sort of like text messages, on their website. This level of interaction is enough to improve your maintenance outcome while saving on unneeded costs. The savings can be more than enough to pay for their service.
As for downsides, what's most often mentioned is speed. Some of their people respond to ticket posts fast, but some don't. If you are always impatient, it might not work for you.
Their prebuy service is very helpful for a newbie owner, especially if you need an inspection by a far-away shop.