My experience is entirely based around using the service through their REST API, so probably not relevant to your case specifically, but perhaps useful for others. $60/yr for unlimited is impossible to beat for my use case (>4TB of data with potential for downloading most of that).
The REST API is dodgy at the best of times. About 1 in 4 requests fail with random errors (Internal Server Error, Timeout, TooManyRequests, Temporarily Unavailable, etc). The API has an aggressive authentication scheme that requires re-auth every 5 minutes or so. Detecting when re-auth is needed is done by waiting for an Unauthorized error on any request. At least, that's what the spec says. What actually happens is that it returns 401 sometimes, and other times it returns a generic 4xx or 5xx class error with the error's message mentioning that the auth token has expired (sometimes the error message even says 401 Unauthorized, even though the actual error code isn't 401).
Upload speed is quite bad, in part due to the constant errors requiring re-tries and backoff, but mostly because the total response round-trip time is slow. This is particularly cumbersome for many small uploads.
Recently I've been getting waves of TooManyRequests errors, where it will only return TooManyRequests for a day, or even a whole week, making the service unavailable to me (currently in contact with them to see if this can get fixed).
So in other words, I've been using the service for about 5 months and managed to upload only ~1TB of data to the drive (on a 20Mb/s up line). From what I hear the official client suffers similar issues in terms of sync-ing speed.
For $60/yr and no real alternative for me, I feel I'm getting what I pay for. I just wish the API wouldn't completely die for days on end.