I can definitely go along with extra batteries: I have two spares, and it's not uncommon to use two in a shooting session. Over time I've learned the obvious shortcuts to extend battery life (I don't leave it on, don't leave it on AF Continuous, power save on the LCD, etc.), and they last a lot longer, but nowhere near the capacity of the much larger batts in my XS-1.
I have the Fuji filter adaptor/hood, and I have a Hoya clear in 52mm to fit it. I also bought the Marumi in 40mm and prefer using it with the hood 'empty'. The Hoya is without a doubt the hardest filter to clean I've ever owned, and damned if I can screw the hood with the filter installed on and off the camera without donating fingerprints most every time . . . The Marumi is fine, the hood screws right into it, just handier for me. And Yes, you will get a shadow in flash pics from the hood at wide angles with the onboard flash. With the Marumi and the hood, I've yet to be able to provoke any vignetting.
I'm old enough to have used 'never-ready' cases with things like Electro 35's and HiMatics, so I got a great Craigslist deal on the Fuji case and use it all the time, as is just gives me a little more to hold on to, as well as protection when it's not in use. And it fits the retro vibe, too.
I picked up both Fuji EF flashes, and they work seamlessly with the X10, fabulous flash results, but the EF42 is bigger than the X10 ! I run rechargeable NiMH AA's in both of them, a big improvement and big savings over endless copper tops. Don't know why I didn't change over sooner, quicker recycling, and I get to walk right by those copper top displays where I threw away SO much money.
The EF20 is a better fit size-wise, but doesn't have the reach of its big brother. They both have bounce heads, and the EF42 head also swivels, really handy for bounce when you throw the camera up on one end to shoot vertical framing.
If you have a leftover screw-in cable release from older film cameras, they work just fine on the X10 (notice the threaded hole in the shutter button?).
I'm going to get the 40mm Marumi polarizer. I'd like a good soft-focus filter for people pics, but will have to get that in 52mm I'm afraid.
All the Best,