I had a M&P 40 and thought it was one of the best shooting pistol in 40 S&W. The felt recoil was like shooting a 9 mm IMO. With that said I got burned out of stocking ammo for every caliber that I owned. I understand that it was a gift and feeling icky about getting rid of it. Maybe just ask her if she minds you trading it for another pistol since both of you are not thrilled about the 40 S&W. Also Storm-Lake has a barrel conversion for the 40 S&W to 9 mm.
I'd throw it in a emergency kit or bailout bag, along with 3-5 mags and a couple hundred rounds of loose ammo. It will work just fine for any handgun needs, and could be handy for arming an other family member even if you have your regular sidearm. Also, having an additional gun in a non-standard caliber may be good if 9mm drys up but .40 is still on the shelves like it did during the last ammo crisis. Also, if it ever gets down to using sub-optimal ammo, the .40 will be a better fight stopper than 9mm with weak target loads.
Don't start buying conversion barrels and mags and such. You are only putting more money into a gun that you don't really like/want. You will regret it. I did that once and regretted it. Also, there is always the fact you are bringing in the potential for malfunctions or accuracy issues with a conversion. Keep the gun like it is and use it as a spare or emergency get to gun or trade/sell it. Don't sink more money into it.
I would not sell it, gift and all, that's me.... I probably would not sink too much money into it either. Buy 300 round of your chosen ammunition for it. Go shoot 100 so you know what you are getting with it, clean it and put it away for a "rainy day" with that left over 200 rounds. Once a year cap off that 200 and replace.
Good advice. I picked it up today and I forgot how puny 9mm looks next to .40 haha. I never liked my .40 handguns because they were compacts (Glock 23, Sig SP2022) but this is a full size M&P. I've shot it before and really don't mind it. I'll keep it as is and tuck it away for just in case scenarios. Like someone else mentioned, it's a standard round without being too standard. It'll still be available when 9mm is cleared off the shelf.
So after shooting it this weekend and really thinking on it, I think it's going to get a light (like my other M&P, probably a TLR-1). It already has night sights. It'll probably get either a FDE or some sort of dark grey color so it's visually very different than my M&P 9 and then it'll get shoved in a bag and forgotten about until I ever need it. I know color coordinating adds money to the gun, but try telling your wife "Hey grab the .40, not the 9mm) and see what happens lol. Saying "Grab they tan handgun" makes things much easier.
Funny, I forgot that I own this gun until I stumbled on this thread again haha It's currently sitting in a bag with spare cleaning supplies, fire making material, spare knife, hatchet, small first aid kit and spare clothes for the family.