It certainly is usable. I'd used Telegram FOSS from F-Droid for quite long before switching to Nekogram (also on F-Droid), a third-party Telegram client with some extra features.

Incidentally, Signal is still not available on F-Droid, and Signal developers are known to be hostile towards third-party clients (for "security reasons." Almost like I've heard that argument before from a certain smartphone manufacturer...)