I just joined WPMUDEV and downloaded the group calendar plugin. it works pretty well, but I noticed some minor defects:
1. new calendar items don't appear in the activity stream. I'm guessing this is because the activity stream is changed in version 1.2.
2. In day view, the month calendar is shown, but there is no longer highlighting days. Either remove the calendar in day view (preferred) or highlight the days that have events on them like normal.
3. in day view, the top date is done properly, whereas the events for date is in the hard to read dd/mm/yy format.
I'd like to make some other usability suggestions/improvements.
1. when you roll over a calendar day, brief info about the items on that day could pop up with event titles, times with perhaps a direct link to the event.
2. The standard view could show two months instead of just one. For example, if the date is march 29, I will only see march, but most of the interesting info I want is in april just a few days away. (widen the grey box so that the 2 months fit without problem.)
3. In day view, why not list the events in a blog style format, rather than needing people to click on an event to see details. It just seems like there are too many steps to get to the content. This is especially the case when you click on the calendar and there is only one event, you need to click again to get the detailed info. (see #5 below for ideas to improve the event display).
4. event links should only be on the event title, not on the time and date, which can just be a light grey. this will help the event title stand out better, and make the date and time easier to read. right now they both just run together and are impossible to style differently with css.
5. the event view could be much better. the h4 tag should be the name of the event, not the words 'Event Detail' Then just put the time and description below it. You don't really need to say 'Description'. The way you are displaying the time and date is Buddypress standard for non-important things such as 'last active'. Because this is an event you should make the date and time display bolder and normal or large font size, and not in that odd orange-yellow style, which mixes metaphors.
6. there is a lot of redundancy in the month view. you have 'Month View: April, 2010' at the top followed by 'April 2010' in the actual calendar then 'Events For April, 2010:' to the right. You probably don't need the first one. I would then move the Prev/year/next links to within the grey box. This will greatly simplify the design. Maybe then bold the date in the little calendar. In year view you could just say: Year 2010. and in day view, as mentioned above, get rid of the calendar, it's odd there.
7. there is no way to turn off the calendar for groups in the settings. once there is a way to turn it off, there should be a setting somewhere to choose if the calendar is on or off by default. (I would prefer off by default, but I understand you want to ship with it on by default).
let me know if you have time to make any of these changes. If not and I can do some of the work, would you include the changes in future plugin versions. (the coding will conform to your style).