Having owned both my thoughts are:
Positives
1.) Front element – The mk1 had a protective glass front element which was a b****r to clean if a stray finger print were to make it on, or if you got watermarks from using it in rainy weather. The new one has a lovely new coating which makes a noticeable difference, water beads off and dries without leaving a mark and it’s easier to clean should it become soiled.
2.) Hood design – WOW what a difference over the old one, I really struggled with attaching the old hood and was always worried about taking paint off which inevitably happens and makes the mk 1's look tatty. Canon has addressed this really well and the new hood locates into a black rubberised groove really easily and will not chip like on the old lens.
3.) Weight – A little less goes a long way, the new lens feels better balanced in the hand with the weight being evenly distributed - the mk1 felt front heavy. I carry the camera and lens on a Black Rapid strap and when swinging it up to shoot, I feel the camera lens mount is having less stress and torque put onto it. With the mk1 I was very conscious of this and would always try to support the lens weight, whereas the new version has me more relaxed about how I handle it to shoot with.
4.) New button layout – Another big improvement, the IS and AF group buttons have now been split out into two panels making blind switching of these much easier. The main winner for me is the relocated and later AF/MF button design which lies flatter and does not get switched over accidently which did happen a few times on my mk1 causing missed shots.
5.) Larger focus ring – I’m not sure if they have changed the gearing on this but first impressions are that it’s easier to MF using this lens, it feels more accurate and precise to me.
6.) Rotating collar – The action on the one is a lot smoother and less notchy, it has soft locating pin which locates every 90 degrees, great for when using on a tripod.
7.) Focus speed – I have mk3 extenders which was a big factor in going for the upgrade. I get faster focus using these on the mk2 lens when compare against the mk1, it’s not night and day but the difference is there to see and I found that there is no noticeable image degradation in the real world using the 1.4x extender and less degradation when utiising the x2. The 1.4x is often found on the lens and have absolutely no hesitation in using it.
8.) Flare resistance - Shooting backlit subjects using the 300 mk2 gave better pleasing images.
9.) 4 Stop IS system - It works well and I hand hold mine all the time getting great images using the x2 extender at 600mm
10.) IS mode 3 - Love it, best of both worlds and helps you maintain good technique rather than relying on the technology to do the work for you.
11.) Image quality - You can't improve upon the 300 mk 1, the mk2 just gets more of them!
Negatives
1.) AF servo accuracy – My first impressions were that I got about 20% less shots in 100% critical focus when compared to my mk1 lens. HOWEVER, Canon addressed this with the later firmware update to bring it back up to the mk 1's high standard, I took my lens in to get this update not long after its release and noticed an improvement. I would say that AF acurracy when utilising the mk3 extenders is improved when using the 300 mk2 as well.
2.) Price – It will cost about 2k to upgrade and if you don’t use extenders I’d probably suggest that it’s not really worth it. If you do have a need for differing focal lengths and extenders are often utilised, then to me...yes...it's worth the upgrade.