My main evidence is introspective, so perhaps not that useful.

MSM definitely changed how I interact on SN in ways I think were toxic.

Pre-MSM

• posted things when I had a cool idea
• checked SN frequently, but not always every day
• zapped things I thought would do well or that caught my personal attention
• commented when I couldn't help myself (conversation was so interesting I just had to jump in)
• kept my total stacked hidden on my profile
• didn't really pay attention to how much other stackers stacked

During MSM

• posted every day, sometimes even when I had to stretch for content (even resorted to recycling some old content)
• checked SN at an unhealthy frequency
• zapped the top 10 or so "all-star" stackers whenever they posted without waiting to read their posts, somewhat often I never got around to reading them
• compelled myself to drop a comment on any thread a thought would be popular even if I didn't have anything that fruitful to say.
• felt grumpy/suspicious about movements on the leaderboard - especially about the hidden stackers do they really deserve that spot? I bet they are gaming it somehow.

All in all, I didn't like how my use of SN was changed by the competition. Maybe I should have stronger principles or more self control or something, but I don't.

I think I will return to my old way of using/enjoying SN, but it's gonna take a little rewiring of my brain.

I totally agree with @grayruby that it could be a great once a year kind of thing.

As I've said before though, SN is one of my favorite places on the internet with and without MSM. So don't take the above as criticism. I wrote it more as documentation of what was going on inside the mind of one stacker.