A user of mine and I have tested KDE Kontact + kcaldav [1] as a client for SOGo. We didn't put that much expectation in it as CalDAV is not yet officially supported in this client but we were very surprised to see that creating an event was enough to make SOGo to crash:

[[
127.0.0.1 - - [06/Oct/2010:18:41:37 GMT] "OPTIONS /SOGo/dav/myuser/Calendar/personal/ HTTP/1.1" 200 0/0 0.004 - - 32K
EXCEPTION: <NSException: 0x33f17f0> NAME:NSInvalidArgumentException REASON:NSDataMalloc(instance) does not recognize stringByEscapingX\
MLString INFO:(nil)
Oct 06 18:41:38 sogod: SOGo watchdog [7888]: <0x0x2454ba0[WOWatchDogChild]> child 9639 exited
Oct 06 18:41:38 sogod: SOGo watchdog [7888]: <0x0x2454ba0[WOWatchDogChild]> (terminated due to signal 6)
Oct 06 18:41:38 sogod: SOGo watchdog [9640]: SNS support disabled.
Oct 06 18:41:38 sogod: SOGo watchdog [7888]: <0x0x241d4f0[WOWatchDog]> child spawned with pid 9640
Oct 06 18:41:38 sogod: SOGo watchdog [9640]: <0x0x27d5b50[SOGoProductLoader]> SOGo products loaded from '/usr/lib/GNUstep/SOGo':
Oct 06 18:41:38 sogod: SOGo watchdog [9640]: <0x0x27d5b50[SOGoProductLoader]> Mailer.SOGo, CommonUI.SOGo, MailPartViewers.SOGo, Admi\
nistrationUI.SOGo, MailerUI.SOGo, Appointments.SOGo, MainUI.SOGo, PreferencesUI.SOGo, SchedulerUI.SOGo, ContactsUI.SOGo, Contacts.SOGo
Oct 06 18:41:38 sogod: SOGo watchdog [9640]: |SOGo| version 1.3.0 (build @vizzini.inverse.ca 201007211512) -- starting
Oct 06 18:41:38 sogod: SOGo watchdog [9640]: |SOGo| vmem size check enabled: shutting down app when vmem > 384 MB
Oct 06 18:41:54 sogod: SOGo watchdog [9640]: <0x0x239c510[SOGoCache]> Cache cleanup interval set every 300.000000 seconds
Oct 06 18:41:54 sogod: SOGo watchdog [9640]: <0x0x239c510[SOGoCache]> Using host(s) 'localhost' as server(s)
2010-10-06 18:41:54.804 sogod[9640] Scanner initialised with nil string
Oct 06 18:41:54 sogod: SOGo watchdog [9640]: [WARN] <0x0x27a6610[SOGoWebDAVAclManager]> entry '{DAV:}write' already exists in DAV perm\
issions table
Oct 06 18:41:54 sogod: SOGo watchdog [9640]: [WARN] <0x0x27a6610[SOGoWebDAVAclManager]> entry '{DAV:}write-properties' already exists \
in DAV permissions table
Oct 06 18:41:54 sogod: SOGo watchdog [9640]: [WARN] <0x0x27a6610[SOGoWebDAVAclManager]> entry '{DAV:}write-content' already exists in \
DAV permissions table
2010-10-06 18:41:54.876 sogod[9640] Note(SoObject): SoDebugKeyLookup is enabled!
2010-10-06 18:41:54.876 sogod[9640] Note(SoObject): SoDebugBaseURL is enabled!
2010-10-06 18:41:54.876 sogod[9640] Note(SoObject): relative base URLs are enabled.
2010-10-06 18:41:54.888 sogod[9640] File NSKeyValueCoding.m: 892. In [SOGoAppointmentInboxFolder -valuesForKeys:] This method is depre\
cated, use -dictionaryWithValuesForKeys:
]]

[1] http://code.google.com/p/kcaldav/wiki/Architecture