From what i see, OP players will not use him as they have other options. However, the people that are usually in the middle of the meta run him with summer kurenai. which is a very annoying combo and there are a lot of different types of teams with these two.

Personally the strongest combo I found with him was as Danzo stated, P1 Kakashi and P2 Kurenai (both Summer) but with Madara Edo Release as the final unit. Literally caused me dread whenever I came across it.

For free players, Kakashi Summer it's the best, because it's a good ninja that you don't need to breakthrough

He seems to be quite helpful in Arena, Especially Themed Arena centered around "Chases".

Even in normal activities he's quite useful, Especially if you're faster than your opponent.

Kakashi is double edged blade, if you are faster in round 2 or you can control your enemy, then he is amazing...
If neither of these 2 requirements can be met, then Kakashi will backfire horribly and eventually kill you...
However, Summer Kakashi with Halloween Kimimaro and GNW Madara (if you have them by any chance) can lead you into a very fatal damage in round 2.
Other option is using Summer Kakashi as support for Rinne Sasuke (if SBed)... With current heavy shield "meta" with GNW Madara and Suigetsu, Sasuke can deal massive damage, ignoring shields, which with Kakashi´s boost can be a deadly combo...

Wait, Is It GNW Madara Or Suigetsu..? That Goes Along With The Summer Kakashi (Variant) And (Rinne Sharingan) Sasuke

I like payslip tesco