Personally, I find great value with Sky Miles, but it's because they are combined with nice DM perks. I have booked dozens of quite cheap Main Cabin tickets where I will automatically get the free Comfort + upgrade, (very much worth it for me as a 6+ ft tall person who enjoys alcoholic beverages) but will nearly always get upgraded to F.

I keep track of my bookings and what the cost I paid in miles was, the cost in C+ was, and the cost in F was. One recent example was roundtrip from JAX to ANC for 55k miles, (including the 15% AMEX discount). Paid the 55k, but C+ was 91k miles which I automatically got upgraded to for free. Roundtrip F was 170k miles. I ended up getting upgraded to F on all 6 legs. Had I paid for F, it was $2,100+ in cash or 170k Sky Miles. So, that ends up being $0.039 per Sky Miles used.

This is pretty typical for me, but I also try to take flights where I am likely to get these upgrades. So, I have a good bit of flexibility and also like to fly later in the day and on weekends. I also do this in conjunction with RUCs and GUCs which then provide even better redemption rates, but again, flexibility is absolutely required.

Obviously YMMV doing this. There's no guarantee to get upgraded to F, but the guaranteed free C+ upgrade is enough value for me even if I don't end up getting upgraded. Otherwise, when not using miles, I just WFBF. But I try not to maintain too many Sky Miles so that's my strategy when using them.