We've used Gitstart to augment our developer bandwidth at codewp.ai for a few months (working on platform rebuild / launch). It's a great offering. The founder, Hamza, and our support team are very responsive and attentive. The outputs for our PRs are great quality. I also like the fact that we're helping real world developers level up their skills without the pain of training them ourselves :).
We're a bit different than some of the companies mentioned here, very small, agile, and quick moving. Was worried that GitStart wouldn't make sense, but in fact, they allowed us to focus the more complex (and fun) aspects of our platform while getting rid of time draws - a great asset for a small startup.
Costs are pretty accurate, communication is good. No complaints at all, they're definitely worth a try.