Our team uses Horde largely for shared calendaring purposes, via Thunderbird. TB is our go-to email client, though I do use the Horde webmail to review admin emails. When workstations have failed and users needed email access immediately, the webmail has been a good short-term backup solution. All have been frustrated with the issues noted below, as well as some daylight savings time quirks where calendar events are mysteriously one hour in error.
Horde is good but not usable as groupware in production, current sme10 version has 2 issues. 1. Event reminders discarded if set to 0 minutes. 2. Event invitations organiser changes to recipient email address and resends invites to all attendees from new organiser email address.