I've been looking at ArcticDesk to replace the WHMCS built in help desk. Only main issue I've had so far was the POP3 email to ticket's download Office365 Exchange. One day I'll get a few minutes to look at it more and maybe get ArcticDesk support involved. So far I do like the responsive back-end on ArcticDesk better then the one included in Kayako. Monthly un-branded license lease at ArcticDesk is another nice perk I like.
Originally I was looking at using Kayako since I used Kayako v3 for a very long time on another project and liked it but had look at other options after Kayako's latest price re-work up of 250% increase (My thread regarding the increase; http://www.webhostingtalk.com/showthread.php?t=1325537 ).