One time, out of the corner of my eye, I noticed some suspicious eye movement (staring at every card) at the next table, and I thought "that might be a card counter." I looked over and yes, it was a card counter, it was someone I knew.
You could play full time for months and never even see someone you suspect of being a winnng player. Surprisingly, this is true even if you only play the best games.
Comparisons
T-H Basic uses the K-O Count and improved basic strategy. T-H Intermediate uses the better BS and about half a dozen more plays than K-O Preferred.