I have used the TPX and the T8, I since sold my TPX and got the SA-17 and absolutly love it. As a pistol is a great optional side arm. I have also done some custom work to make a mod kit to make my SA-17 into a nice little light sniper.
The ease of use and not having magazines is also much easier. Its faster to reload than my TPX ever was, also I have never broken a ball in it yet. The sizing kit you get with the rifle upgrade kit is nice but it does need more sleeves, as there are more than 3 variations in size of a paintball.
Over all I do like the SA-17 for my style of play. Also the thread and sleeve set up is much better for switching to different types of barrels. I currently have 4 different barrel lengths including the stock barrel, that alone makes it much more versital than other pistols I have used.
As to its size, I'm a larger person than most of the people I play with and against and its almost a perfect fit for the size of my hands.
Also the maintanence of it is so much easier than my TPX ever was. One pin and the whole assembly slides out for cleaning or checking the orings or just simply reoiling the bolts.
All in all I think its a very good sidearm, and with a 6 to 10 inch barrel its very accurate for quite a distance.