I’m not sure what kind of experience you will have with yours but I can give you a little insight on what I have had to deal with so far on mine.
The first thing out of the gate to mention is the .25 Impacts are tuned from the factory to shoot the JSB 25.39 grain pellets. Therefore, I am in the process of tuning mine to shoot the heavier 33.95 grain. So far I have tried the MKI and MKII and found that the MKII seem to load a little more smoothly than the MKIs. I am still trying to work out the best reg pressure-hammer spring tension and valve spring tension to get the best accuracy and shot count out of the rifle but the weather where I live has put a hold on such activities for shooting outside. I’m gonna have to wait for warmer weather to continue this process. I have purchased one of the ED Gun electronic gauges to aid in this process which should help me save a ton of pellets and some guess work with the tuning. It might be something to look into.
The second thing I noticed was my barrel lock down screw has a tendency to want to back out over time so I am constantly checking that to make sure it is snug before shooting. I might need to try the modification that Ernest has shown on his you tube channel regarding this issue.
The third thing I have come across is the pellet probe grub screw being loose causing the pellet probe to bobble around a little bit. I ended up removing the screw and slapped on some blue lock-tite to keep that in place and have not had an issue since.
The fourth thing was the magazine that comes with the rifle. I had to “tune” mine a bit because the spring tension was preventing the pellets from advancing in the mag properly. Since I tweeked the mag it functions like it was meant to. I also ended up purchasing 2 of the side-shot 26 round mags and have not had a single issue with these except for blowing through a whole tin of 300 pellets before I realized it
And last but not least I would highly recommend getting a different pistol grip, one that provides some material where the webbing between your thumb and index finger meet because I found after hunting with the rifle the machined grooves in that area have a tendancy to remove skin from that part of your hand, kinda like sandpaper.
That is all I can think of for now. Good luck with your new purchase!