I've bought my first BB gun (an Umarex M&P 40) 5 weeks ago.
I received my G19 last friday... All of this is pretty new to me. (Yeah, I bought two guns in five weeks... )

I've loved the M&P 40 as soon as I handled it in my hands the first time. I'm still looking for the best way to place my hands, my fingers, etc. It's a work in progress.
I've already fired almost 1500 BBs with it. I'm getting better and better... It's a really nice gun.

Concerning the G19, it is quite the opposite.
This gun literally tortured me. The front and the rear texture of the grip hurt my hand so much, I wasn't able to hold it firmly.
The trigger guard as well hurt my fingers. And the trigger itself was like a set of blades, every edge was kinda sharp.
Everything about this pistol felt unatural and uncomfortable in my hands.
After a few magazines, all the pleasure was definitely gone.

I asked myself why a so popular gun could be so painful to hold?
So I made a few searchs on Google and I've discovered that so many people are complaining about it. (The texture damages not only the hands but also the clothes...)
A lot of grips enhancers exist : Talon Grip, Decal Grip, Foxx Grip,...
But you have to take off the back of the grip to load the C02 cartridge... and they are quite expensive.
Then, I found posts and videos about people stippling, Dremeling and sanding it...

So, I've decided to do some careful Dremeling and sanding. (I had nothing to loose, I mean... I hated the gun...)
I removed all the front and back texture. I made a undercut on the trigger, I can now have a higher grip and my fingers fit perfectly in the grooves (which I have enlarged a bit).
I sanded the edges of the trigger and the trigger itself. All this process took me a lot of time and patience. I did it in three sessions with shooting tests between them.
And now, it's just "waow"... really. Visually, it's not perfect and the sanded parts are lighter, but I don't care because now I can shoot one hour straight and have a lot of fun, it's like a new gun, it fits perfectly in my hands.

Other CON'S :

1) The second safety (top of the trigger) engages accidentally every 5-6 shots (I'm right handed). Very annoying... It's useless, it's a dual action only pistol, the trigger is long and pretty hard to push. It's impossible to fire the gun by accident.
2) I miss the blowback feel. I think it's gonna be my first and last non blowback pistol.

The PRO's about this gun :

1) The magazine is so easy to reload (comparing with the M&P 40 whis is unbelievably hard).
2) The cartridge installation is very easy too (easier than the M&P 40 because you don't have to take off the holding screw completely).
3) The gun is very powerful. I use a collapsible pellet trap (Crosman) as target and there are 3 layers of curtains inside. The 1st layer (the thin red one) was intact after shooting 1500 BBs with the M&P 40. After shooting 10 magazine with the G19 it was completly shattered and some BBs even went throught the second layer.
4) I love the sights. I like the "U" thing.
5) It's cheap (89$ CAD).
6) You can shoot more than 5 magazines with a CO2 cartridge.