We always do thank you notes. Always. I want my guests to know that we appreciated the gift, but we appreciated their presence even more.
When they were younger I wrote them and they colored them. Then as they got older I wrote them and they told we what to say and they signed them. Last year, my son was just getting to where he was writing sentences well. So he wrote a form letter of sorts and I copied it for everyone, he signed each one. This year, he will be stuck writing each one on his own.
And I don't judge anyone or get upset when we don't get thank you cards, I realize that people just busy or don't out much importance on it. But I have to admit, I notice.