I don't have any experience with the X-bolt, but a friend of mine has the A-bolt. It is a very accurate production gun. I would say with good handloads, a guy should be able to achieve sub 1moa groups (3 shots). I think anything more than 3 shots would suffer from a warm barrel.
I have the x-bolt, but not a 280. A .300WSM in composite stalker. Great all around gun. There is almost no aftermarket support for them, but... it shoots pretty darn good out of the box. A custom rifle challenger? Prolly not. But a ready to roll hunter, absolutely. I'm still working up loads. Was disappointed in my first few, but a "thrown together" 180 E-tip load started my season off rather optimistic so far... 3 in 1 ragged hole with 2 more pushing the group to nearly 1" at 100.
Very little aftermarket parts, if you plan to "upgrade" as you go.
NO aftermarket long action scope bases. There are some shorts though.
Sloppy action / overly loose bolt, if you ask me.
Not many people around here shooting them... so not a TON of info floating.
Trigger very nice, but only goes down to 3 lbs. (One aftermarket spring (Ernie the Gunsmith) is nice, but the trigger will break on a hard close on anything under 3... at least mine did) Of course... no aftermarket trigger options.
I too have an X Bolt Composite Stalker in 300 WSM and it's a shooter! It will consistently put 5 shots into 3/4 of an inch at 100 yards and I'm sure it could do better than that if I didn't have problems holding her rock solid. When I shoot from the bench the narrow fore-end and the lack of weight in the gun make it hard for me to hold as steady as my varmint rifles, but I bought her to hunt high country deer and elk and honestly for a factory rifle at around 700 bucks I don't think there is a better one out there.
I love the swelled palm (only for right handed shooters) and the trigger-while not super light-feels awesome at 3 pounds. Like bs911 said there is basically no aftermarket for them, but I bought mine because I liked the way it felt right out of the box and it's shot well enough I don't have any desire to change anything. The only modification I had to make was to grind a little bit out of the front of the magazine so that my 185 grain Bergers would fit in the magazine without getting stuck.
Also the recoil pad seriously takes the felt recoil down immensely and I have no problem putting 30-40 rounds through it in one range session.
If you want the ability to customize your rifles then I'd stay away from Brownings in general because there are so many cheaper options if you shoot Remington or Savage, but I've bought a few Brownings (A Bolt, X Bolt, T Bolt, and a BL 22) simply because they felt like I wanted them to for their intended purpose and they've all shot great.
Good items to mention BrowningLover. I agree that the factory stock and recoil pad are very effective. We do load development on a lead sled, but even without, the recoil is not all that bad. Not enough to make me want a muzzle brake. The stock itself has a (subjectively) superb feel, even walking through the woods in the cold rain and snow. I did have to sand out the fore-end on mine a tad. After about dozen or so shots, the barrel was starting to touch.
I'm getting ready to do a grind job on my magazine as well to accomodate some longer Bergers. Question... did you have to channel out just the mag itself, or did you also have to cut a little around the floorplate area (?) so the round could clear out of the magazine? Looks on mine like the mag and action mate pretty close. Haven't started the loads yet, but I don't need a whole lot more than the mag will accomodate already to reach the lands.
I've only had problems with the shells coming out of the magazine once or twice but it's enough to worry me about a failure during hunting season so I'm going to take my gun to a machine shop and have them open up the very bottom of the receiver with a 1/16 inch end mill and that will allow the very tip of the Bergers to clear.
I've probably shot 100 shells through my gun after opening up the magazine and I think I've only had 3 failures to feed, all I had to do was push the back of the shell down into the clip and it allowed the bullet to clear the bottom of the receiver and fed fine after that. Also, every failure to feed was on the first round out of the mag.
I'm on the other end. I don't have an x-bolt, but I do own an a-bolt II in 280. I just got it back out of the safe after five years of not shooting it. To my surprise it shot a .5 @100 with factory 140 ballistic tips. I went back home and started reloading. Went back to the range with my loads, 150 ballistic tips and H4831sc loaded .010 off lands. One ragged hole with 3 shot groups. Think she might get out of the safe just a little more this deer season. Good luck on the new purchase if you decide to get one.