Hi Pierre,
The column grouping doesn't work well enough for day to day usage. As soon as I group several sets of columns, things start to get weird.
Apart from that, the feature has a lot of potential. I like it -- I don't know if others have had the courage to test it, but it's fun and practical. A bit like the field hierarchies, it's great to group sub-columns under one main column and only expend it when needed. But since doing all that convenient grouping takes time (moving the columns around, creating groups etc.) it needs to be robust.
I'll try to be as precise as possible, but I don't have enough time to test thoroughly.
1- Sample DB, invoice grid
2- Select : InvoiceID, From, To : group under InvoiceID
3- Select : HrsRate, Duration, Billable : group under HrsRate