I have tested SAP GUI 6.2 as well as some of the HTML and ECatt. You may run into a couple problems but there is only one that may be a show stopper that I can think of off hand.
In transactions such as ME21N there are checkeditboxs that are really narrow. During replay testpartner instead of clicking into the correct field can click the expansion button for the checkedit to it's left. This causes a window to appear and the script to go haywire. This is supposed to be fixed in the SP that should be available any day now for TP 5.3.0. but I will have to test it to be sure.
You will also probably find that transactions that have a large number of controls on them replay rather slowly. This is because TP has to enumerate every control and takes some time. I can live with the slowness as long as it replays correctly.
Testing ECatt is completely different and you may find it cumbersome, but once you get the hang of it, it's ok. You have to check blobs in and out from ECatt to TestPartner and vice-versa.