How good is Janus as jungler?

I feel that he has good potential in the role but I'm not certain.

Not my preference. Although he's got great mobility, I just don't see him as being the best ganker. Outside of his 1/2 combo he doesn't have a lot of burst, and once those are down, he's got nothing. Instead, I really think his place is just Mid...and he can use that ult to move from mid fairly quickly to nearby jungle areas, or as far as one of the other lanes...and if he can join in with the jungler in a gank, great.

Ok thanks.

He is viable as a jungler but I don't recommend playing him there.

Gonna agree with Bran he pretty much only has his 1-2 combo and if you use it to pop from a wall you only have your 2. Also most magical junglers do struggle a little bit with early clear IMO (because of how reliant you are on basic attacks during that time or you're gonna run out of mana quickly).

We forgetting his ult?

Mobility wise good point: saves you from needing to use portal to get through a wall while ganking, however that is only ever ~2 minutes, it can give the gank away if used from to far and overall (unless from point blank) hard to hit.

from a ganking viewpoint in most cases you won't have it up as much. It does make a significant difference in threat if you have it up, however this counts for most junglers. However they are not as reliant on their combo as janus 1-2, mostly caused by their gap closers or easier to hit abilities if you ask me.