I don't use a pass-thru simply because I don't have the need. But there are basically two type of pass-thru.
One has a battery and one doesn't. The one with the battery you can vape while connected to the charging cable while at your desk and then disconnect it and take it on the go with you. It's not a 'true' pass-thru as you are really using the battery power and your USB cable is charging the battery as you vape.
The other doesn't have a battery and won't work unless you have it plugged in.
Of course the one with the battery is much more versatile but comes with a slightly higher price.
As for decent ones, I will have to defer to the more experience vapers with this type of
device. I've often thought I should buy one as a backup to my backup to my backups - so I'll be watching for recommendations.