Compared to other services on the market, Uservoice's main differentiator is the simplicity of the feedback and support mechanisms. The widget that is installed on your website only has two options for the users: Contact Support Submit feedback Users can also quickly see existing suppo...
It includes unlimited number of users, agents, topics, support of SSL encryption, Polls, Announcements, and 1 year of Support & Upgrade. There are also commercial add-ons that should be purchased apart from package
You'd think it would be the best Salesforce integration ever since it's owned by Salesforce, but it's completely sub-par. You have to have customer service reps manually connect SF accounts to Desk Companies.
PHPBack is fairly young (first stable version was releases in July 2015). Currently the community is small, which may make it more difficult to find help/support. However, the developer is active on the github page and generally responds quickly.
Viima does not offer many ticketing and customer support features. Instead, it only supplies a way for users to add suggestions and other users to provide their feedback on them. This may be too limiting for companies looking for a full user support service.