Mine (4 & 6) have been to lessons since 5 months. They go to a council pool, so considerably cheaper than some private lessons, and in league with other activities for children (£19 per month). I've been happy with it as an activity for them to do, with water confidence and early foundations for swimming as a bonus.
At 4, they get to the foundation stage where swimming is taught over just kicking around and being supported by the parent. DS1 got stuck for 18m at this point, partly because he was small and found the depth and grip on the floats difficult, then because his attendance was affected by a change in my frequency of meetings. At 5/6, he's progressed through stage 1 much quicker.
We did an intensive course for a week to give him a boost. They were split between those nearly ready to move up, and complete newbies who needed guiding into the water. The progress in 5 days was impressive.
I've never regretted the lessons as we've enjoyed them, and it's made us get to the pool weekly, which we'd probably have struggled to do casually. On holiday last year, the pool was slightly out of DS1's depth, but he and we were happy for him to swim around freely with floats (closely supervised!). It's at times like that that I feel there's been more of an advantage to all those hours waist deep in water with them 
Swimming lessons before 5 don't have a massive benefit towards long term swimming ability and aren't worth paying considerably more than any other activity for a reluctant child.