If you're new to the X series, please note that you can't copy&paste the .DAT firmware file directly to your camera's SD card. You either have to use an internal or external card reader or to use the FPVUpdater software to transfer the file to the SD card (you can find this program via Google). The first solution is the better one, because the updater software will only be able to transfer the body firmware, not the lens firmware, and you have to update BOTH of them in order to benefit from the improvements.
When you've installed both body and lens 2.0 firmwares, the X-Pro1 performances will improve on three counts.
1 - Better AF
Fujifilm X cameras aren't quick shooters. It took a big firmware upgrade to get the X100 up and running properly. Now it's the X-Pro1's turn, and not a second too late, because the AF was really lacking precision and speed, more so than the X100 in my opinion.
The 2.0 improves the speed of auto-focusing. Granted, you still won't be able to cover Indy 500 with your X-Pro1, but it's not that type of camera anyway. In bright sun light, you can now expect almost instantaneous focusing, and improved speed in the shade or indoors.
The precision has also been substantially improved. Like the X100 in its infancy, the early-firmware X-Pro1 was prone to focus on the wrong element of the frame, even when you used the center point. My first impressions of the 2.0 firmware is that the AF was dead on 95% of the time.
Obviously, these changes are immediately apparent in the electronic viewfinder, but they're also important when using the hybrid optical viewfinder, because you can hear and feel that the focusing is quicker, and you know that it's more precise. Hence, because you don't have to worry that much anymore that the AF will spoil the shot, taking pictures with the optical viewfinder is a more intuitive and fun experience, moving one step closer to shooting with a film rangefinder.
2 - Better MF
Manual focusing on the X100 and X-Pro1 was a big let down. Actually, when you properly update, set up and know how to operate the X100, the only serious issue that remains is the frustrating manual focusing. Why is that so? Because it takes endless turns of the focusing wheel to get from distant to near focusing, making it far too tedious.
The 2.0 firmware has greatly shorten the number of turns necessary. At last, manual focusing the 35mm lens feels like manual focusing any other DSLR lens. Hurray for that.
Also, the 2.0 firmware adds another level of digital zooming to help with the focusing. That's neat, if like me, you're a bit obsessed with clinical focusing.
3 - Better write-to-card times
The write times were ok if you shoot JPEG, but when you shoot RAW, and especially when you're stupid enough to shoot RAW in continuous burst mode like I did for my carnival pics, the buffering delays were bound to be frustrating.
It's quicker now, but if you shoot RAW, I would still advise to get a
Sandisk Extreme Pro card, in order to get the best from the X-Pro1.
Conclusion
All in all, a beefy upgrade and a vast improvement of an already excellent camera. Like for the X100, don't judge the X-Pro1 on the experience you or other people might have had using the early-straight-from-factory firmwares. Just like the X100 is now a totally different camera from the one that's featured in the early reviews, the X-Pro1 almost feels like a new product, and an amazing one at that.