We've been using it for the last week and I've personally found it pretty flawless so far.
The only complaint I've heard from other people is that they couldn't sign-up when their phone was offline. But not a big deal given the security advantage of routing through a local device.
That is Telegram's biggest security/UX compromise which I hope they address, since I'm sure 90% of people using Telegram aren't really using E2E encryption the majority of the time - due to the fact encryption is not enabled by default in the UI, the lack of syncing conversations between devices for "secret" chats, and the fact a few official Telegram desktop GUI clients don't even support secret chats at all (including on Linux).