This is, mechanically, among the best retractible gel pens I've used. I don't use it only with Pilot refills, but also other gel and even ballpoint refills.
My first reaction to this is that it's expensive. Here it costs twice as much as a G-2ex and considering the refill is 1/3rd or 1/6th of the price, the difference is huge for a pen which is basically rather similar except for design. But yes, it is more comfortable to write with and there are internal refinements to it as well, as the spring which keeps the button from flopping around.
Is it worth it? If you want the slightly larger diameter and the comfort of the sort of semi-recessed clip it can be, especially if you write so much the cost of the refills overshadow the pen cost in the long run (which indeed is very likely when the difference is about three refills and if last on the order of a month).
Unfortunatly, it's not better at everything. The clip sits lower, so in some pockets with flaps it won't ride as well. Not that the difference is large. More interestingly, the same gel refills won't fit in it as in a G-2ex. Zebra refills fits in both, but Niji's are too wide around the tip to fin in the Dr. Grip Gel.
After getting a Pilot BPS-GP XB (1.6 mm) ballpoint and noting how smoothly it wrote, I got the idea of fitting the refill into a larger, retractible pen. Both because it writes about as well as a gel refill and because the ballpoint refills for the corresponding models are all unneccessarily quite tiny. It doesn't engage the spring, so it's got to wait for some other time.
Instead I arranged it so I can put Parker or Ballograf ballpoint refills in the Dr. Grip Gel. It's easy, all it takes is a 6x14-15 mm cylinder. For example you can use a cut down Fisher refill adapter. This will give you a much smaller force in the retraction mechanism; 14 mm gives about half the force compared to the original, or ~2.5 N, because the spring doesn't engage as far down. You may like it, but for me it was a bit too low, so I rectified it by using two springs, the second a smaller diameter one I took from the pen which came with the ballpoint refill, which brought it up to about 4.5 N.
You can compare it to the G-2 and G-2ex pens.
Mass 20 g Tip-centre of mass 65 mm Diameter ~60 mm from tip 13.8 mm Pushbutton force ~5 N Total length 142 mm Protusion over clip 22 mm
Pilot's web page about gel ink pens.