I bought them last week and have run twice in them. One fast hill session on a treadmill and my final long run pre-Boston on Saturday, which was 22 miles with 7 at marathon pace.
For reference I'm comparing them against Adios which are my marathon shoes of choice.
I'm really in two minds as to whether to wear the Sub2s at Boston or not. Firstly, they are LIGHT. Noticeably lighter than the Adios. They also look great. No complaints about the fit - very similar to the Adios.
The primary difference to the Adios is the ride. It is definitely firmer. There isn't the 'squeeze' of traditional boost. Over the course of my long run, I thought they were probably harder on my legs than my Adios would have been.
Looking at my stats post-run, it was noticeable how the marathon-pace part of my run was at a lower heart rate (by about 5-7bpm) than I would have expected. Obviously this could have been other factors, but I think the lightness of the shoe played its part.
So it's a toss-up for Boston. The relative protection of Adios and traditional boost, vs. the potential speed benefits of the Sub2.
I switched to Vaporfly 4% before the New York marathon last year and regretted it in the race. However, the Sub2 is a much more similar shoe to the Adios, so it is less of a change. But still, I know changing shoe a couple of weeks before a big race is never the best idea. Decisions!