I'm using the Sentry free tier and it is great. The main value is how to managed repeated errors. You can group different exceptions for the "same" error, resolve errors that have been fixed or ignore known errors until they occur to more users, more often or what have you.
If you are good about squashing errors you can make it very far on the free plan. Plus they have some burst detection built in. Just make sure that "expected" errors aren't just ignored in the UI, stop emitting them in the app itself so they don't count towards your quota (and it keeps your logs tidy).
I haven't been using their tracing or anything because their Rust SDK doesn't seem to support it despite claiming that it does (or I have set it up wrong).