I get a lot of value out of Aeroplan, which is why I like Membership Rewards. The business class redemption rates are better than United, and a number of airlines don't pass along fuel surcharges. I'm not a big fan of United's program, given the expensive redemption rates for partner awards.
I'll agree it can be a challenge to get value out of SkyMiles. The one big redemption I made with them was 160,000 miles for a R/T business class award, which was much more than I wanted to spend, but I didn't have many options, since my dates weren't flexible.
With more and more commonality among programs (for instance, Chase adding Flying Blue), the best option for me is to pick the highest earn rate for each purchase regardless of the program. Even though my MR balance is the highest among the three programs, I'm not specifically loyal to it - it just happens that I can earn the most MR points day-to-day.