I'd heard of OGO on and off for some years, but never looked at it seriously (I've been a Lotus Notes administrator then developer for over 10 years). However, since I started to look at it a month or so ago, I've come to realise that as groupware OGO offers very many of the features that Notes offers. For sure, OGO is not going to be as scalable as Notes (maybe SOGO is), but because OGO implements WebDAV, groupDAV and xml-rpc interfaces it is possible to develop your own application front-ends to the core API (just as you can in Notes).
What put me off OGO for a long time was the default interface shown in the screenshot linked above these comments. Well, first let me say, you can only get an idea of how cleverly implemented OGO is when you start to work with it. For example, the IMAP mail client implemented as a web app is the most fully-functional IMAP webmail client I've seen. There are large ISPs who offer an IMAP web-client that is positively stone-age compared to OGO's implementation.
To really get an idea of how good OGO is, you must install it and play around with it for at least a few hours. Then you will start to see how flexible it is and how well-integrated the various components are (mail, document-management, project-management, tasks, contacts, calendaring).
I would suggest starting by getting the InstantOGO cd image and installing it inside a virtual machine (VirtualBox worked best for me). In terms of reading to orient yourself as to what to expect, get the brief user manual: http://www.opengroupware.org/en/users/docs/manual/index.html . Finally, if you decide to install it for yourself, expect to spend far more time twiddling with Postfix/Cyrus getting them to work right than you will spend getting OGO to work. If that all sounds like too much work, then just buy a license for InstantOGO.
The people on the mailing list are very knowledgeable and very helpful. When I was struggling getting part of my install to work, one guy even produced a new set of packages to get me going. Clearly there is a lot of passion here - where else do you see such incredible support?
Finally, don't put off evaluating OGO just because you don't like the default web UI. Once you've got it up and running, each user you create can select one of several different skins, some of which are far more aesthetically appealing (well, to me anyway).