We bought those red doohickeys, too. They are much easier to carry around than the long 2x6's we used to use.
We also added a 4-pt leveling system (ground control) just over a year ago [LOVE IT], so technically we don't NEED to use these, but in one spot we go back to often (7 weekends a year), the hole on one side is deep enough that we would be picking the tires about 2-3" off the ground... and we're just not comfortable enough to do that without a backup system (i.e. leveling blocks).
Our axles are close enough together we had to trim the tip of the red ramps to get everything to fit, but they do work well.
We have also run into the scare of getting them un-blocked without losing an arm... but a long-handled sledge hammer took my arm out of the hazard zone and did the trick.
So, on the whole, we've been happy with them.