I just stick a towel on the bottom of the tub just before my kids get in. Works fine for us - suction-cup bathmats are beyond gross!

 

I must've had a sticky kid.... he was fine .... I had a baby seat for him when he was little but after that we never had a problem...

 

It probably makes a difference what material your tub is made out of. Ours is the old slippery metal kind and the suction cups work fine. The mat I have is kind of spongy and has holes so air gets underneith. It also can be washed in the washing machine so it's easy to keep from getting gross.

 

you can get grips that are like stickers and just glue to the bottom of the tub, flush, so nothing to get underneath and gunky.