CMSWatch.com have recently published report of 30 ECM products including SharePoint, Documentum, Nuxeo & Alfresco. I haven't read it yet (need to convince my boss first).
I've used/developed Documentum and SharePoint but have had limited exposure to Alfresco. What I've seen so far is very impressive though:
Performance - it's like light speed compared to Documentum Webtop
CIFS support - great feature, much better than WEBDAV, zero client footprint as well!
Smart spaces - really like this, there's no equivalent in MOSS/Webtop
Usability - nice default user interface, much better layout out than Webtop/MOSS
Javascript API - very nice. You can invoke custom code in Documentum via the Business Objects Framework, the Method Server, DocBasic, etc but you need to develop/deploy JARs/scripts server side so its more effort and not as flexible. You can override the document lifecycle in MOSS as well but be prepared to develop/deploy. MOSS doesn't support a scripting language but I guess you could integrate BOO if you were smart and determined.
DM seems very well executed!
Stuff that could be better:
WCM - still haven't got my head around this yet. Seems to me that Alfresco facilitates generation of static content and that you have to write your own components in whatever language you prefer for dynamic behaviour. This appears to be the opposite of MOSS where you build a dynamic website using site content/components (i.e. MOSS WCM sites sit on top of the repository).
Personalisation - is there a way for the user to set the default view type (i.e. detailed rather than icons)?
Accessibility - had a look at the Alfresco 1.4 markup in the Community Site. Aargh - reminds me of SharePoint! Needless to say to failed validation.
Configuration - OK it's Spring so XML it is - I get it and yes it's very flexible but reloading the webapp to implement changes seems like taking a step back. Point and click would also make life easier for support staff. How about using Groovy? I thought that Spring+Groovy could be used for reloadable beans?
The finishing touches - the core repository is very innovative but I'm waiting to see if Alfresco will polish off the product with stuff that I'm used to in Documentum (e.g. comprehensive value assistance, virtual documents, implementing repository configuration changes without restarting the webapp - data dictionary, scheduled jobs, etc).
Community - incredible though it seems there is more chance of getting free online assistance for Documentum and SharePoint issues than for Alfresco. I am very impressed that members of Alfresco respond to some of the posts (especially given that support is one of their revenue streams). Having said that, I am a little concerned to see so many unanswered posts on the forums.
Documentation - it's not like there's nothing but there could be more and it could be structured better. For example, the online docs for the Spring Framework are very nice. The online Django book is another great example.
XForms for DM - yes, I realise that I'm not the first to mention this.
I hope that some of the above features start dropping in once Alfresco have finished with their major architectural changes (AVM, etc).