With the exception of sitepanel (never used it, but I know the devloper who is a pretty solid php developer)..... they are pretty much all crap.
Kayako slows as your db grows till it pretty much caves. Earlier we migrated alot of our clients from the USA servers over to London, and naturally when you move 400 odd people over to a new server - a large proportion dont read their email + our sales tickets + renewall queries + normal ticket stats = Kayako being an unhappy bunny.
We are in the final processes of taking over another host with a similar number of customers, and im sure Kayako will love that too
There is another player coming into the industry (should have been out earlier this month), who will blow them all away as the code's rock solid and I for one will be upgrading to it