Sirui T-1204x or T-1205x

Like Anders W I own the T-1205X.  I bought it to replace the Sirui T-025 which due to the fixed vertical column wasn't particularly stable in windy conditions.  I can't speak about a comparision with the T-1204X through personal experience as I haven't had one, but I can give you my thoughts on some of your questions anyway.

Yes, certainly.  One twist with a wide hand covering all of the twist locks will undo them all.  Occasionally I wish for a bigger hand for this approach - the four-leg version would be slightly better.

I actually kept the ballhead from the T-025, the Sirui C-10.  It's rather lighter than the G-10 bundled with the tripod and shorter too.  It does away with the friction knob, using the main locking knob to fulfil that role.  It has a huge amount of grip, no slipping at all, and is very smooth in its action.

The Sirui heads are excellent.  A recent German test of ballheads ranked the Feisol CB-30C top (88 out of 100), the Sirui G-10 second (82), the G-20 third (81) and the C-10 fourth (75) in the "under 100 Euros" category.  The test, in German, of the under 100 Euros category can be read here: 
http://www.traumflieger.de/desktop/ballhead/ballheads2.php

It comes with a decent quality bag with a good strap.  I do know the problem though - when I've taken the tripod on a shoot I've ended up carrying it over my shoulder as well as the camera bag.  I might get a slightly larger bag with tripod straps underneath.

Yes indeed.  It is stable, and as light and small as to be expected for the degree of stability on offer.  I don't have it with me all the time though - the T-025 was better for portability due mainly to the thinner leg sections.  The T-1205X comes in a surprisingly big box and the carrying bag is a fair size too, at least compared to the T-025.

Michael