I've been using it for a few weeks and, quite honestly, it's not so good for marketers. Like you mentioned, it's not the easiest to use for someone who doesn't already have the technical skills.
Where this is really useful is on the agency side. It can be really difficult to get clients to update their tagging. They have a lot going on and changing a conversion code sometimes gets pushed down the list of things to do. GTM allows that person who knows a bit of the technical background but doesn't have the ability to change the code to go in there and make updates.
There are a number of better options--Satellite is one--but of course none of them beat Google on price. If you are reliant on Dart, GA, and AdWords and don't need to work with advanced rules or events, GTM is great. Otherwise, you're likely better off looking elsewhere.
I think the initial implementation can be tricky (we'd recommend a Google Analytics Certified Partner to help), though once that is done, our initial experience is that adding tags is not too difficult.
We can help with updating tags (please reach out to your Account Manager), or please reach out to me in my profile.
Where this is really useful is on the agency side. It can be really difficult to get clients to update their tagging. They have a lot going on and changing a conversion code sometimes gets pushed down the list of things to do. GTM allows that person who knows a bit of the technical background but doesn't have the ability to change the code to go in there and make updates.
There are a number of better options--Satellite is one--but of course none of them beat Google on price. If you are reliant on Dart, GA, and AdWords and don't need to work with advanced rules or events, GTM is great. Otherwise, you're likely better off looking elsewhere.