The best parts about it are that the source is very readable and it's extremely easy to override the default behavior of many modules. I would strongly recommend doing this as much as you need to because Kohana's defaults may not make sense for your usecase. The biggest downside is that v3 is very different from v2 and a lot of the documentation and help still assumes v2 (this is why easy to read source code is important.
The best parts about it are that the source is very readable and it's extremely easy to override the default behavior of many modules. I would strongly recommend doing this as much as you need to because Kohana's defaults may not make sense for your usecase. The biggest downside is that v3 is very different from v2 and a lot of the documentation and help still assumes v2 (this is why easy to read source code is important.