I don't have one; can't afford it, and bull pups are not my kind of rifles. However, I am familiar with a couple of owners. Part of your pellet choice is going to be what you want to do with it. If your use is for 75 to 100 yards and beyond, the the JSB King Heavies are a better choice. For closer general shooting, the JSB Kings are also very good and a bit cheaper. Ted Biers, shot the JSB King Heavies last fall to win the Extreme Benchrest match. The final is shot at 100 yards.
The one thing that the FX Impact is not, and why there are a number of bad reviews, a beginner's gun who wants something to shoot out of the box. It is a self tuner's rifle. The thing is somewhat complicated, and the manual must be read, understood and followed regarding tuning procedures. Those bad reviews have been from folks who really should not own one of these.
However, being a tuner's rifle, one can get just about any quality pellet that fits the bore to shoot incredibly well. But, to do so will take a lot of shooting, and tweaking. Each new lot of pellets may also require a slight tweak to get the performance where one wants as well. Keep accurate notes on what you change and what the effects were.
There is a lot more to tuning than consistent velocity in the range that the pellets like. There is a significant factor in getting the barrel and action vibrations to be minimized at the time the pellet leaves. Also, you want to consider shot count per fill. The lighter pellets generally will allow more shots per fill. High velocities are not your friend in general. The thing will be capable of pushing pellets way too fast, but also too slow to be of much use too.