I have problem with grid rows groupping in 4.2.0 beta.
Store config contains "groupField: 'DateCreate'" where DateCreate is DateTime field with empty time, like this:
Date {Sun Feb 17 2013 00:00:00 GMT+0600 (Ekaterinburg Standard Time)}
and for rows with identically date creating separate groups (with same names).
Another bug - getGroupString working strange. If I assign to store getGroupString function:

getGroupString: function (instance) {
return 'A';

- anyway creating multiple groups (instead one "A").
In 4.1.3 all working fine (but with a little visual bug with column resize + grouping - column header width != column width after manual resize).