This might be common knowledge by now considering how easy it is to find this repository, but it’s new to me so I’ll share: apparently, the desktop app developer is actively working on rewriting their app using tdlib.
Telegram Desktop would be the first official app to use tdlib. Right now, the only telegram-related app that uses it is Telegram X, which is an experimental app. The bot API, though, has always ran on tdlib since day one.
Yes, using tdlib is almost always preferable to using a custom MTProto implementation. tdlib is guaranteed to implement the encryption protocol the correct way and it will also handle cached data so you don’t have to. Will also save some time with the development and most importantly it abstracts some of the weirdness of the API so you won’t have to deal with them. Also it’s likely that a custom implementation won’t be as performant as tdlib.
Unluckily it reached maturity relatively recently, so no official app has been able to move to tdlib yet (except for Unigram, which is an almost-official app built for Windows, that was initially using its own implementation but moved to tdlib some years ago)