Unless I am mistaken, the 8.3 has a mechanical locking suspension. You need to unload the front (by getting off the bike) to turn the knob to lock/unlock.

The 8.4 (at least the 2013 model that I have), has a hydraulic locking suspension, which means you can turn the knob at any time to lock it (since it just closes a hydraulic valve, and allows variable firmness). I just reach down while riding to twist the knob ... not hard to do. They did however switch from the to SR Suntour NRX to the RST Nova for 2014 so you may want to double check

The 8.5 has the remote switch.

I like it locked on the road, and unlocked on the trails to smoothen things out. Since I bike on the road to the trails, I do twist it while riding just before hitting the trails. You may also want to lock off road when power climbing to avoid bobbing the front. I don't think the avoidance to reach down is worth $300 ... then again you also get the iso-zone rear dampeners, and better crank and derailleurs as well for that upgrade price.