I wear my tuckable both tucked and untucked. It's an incredibly comfortable holster either way, especially considering I have a G19 riding in it. Untucked, I can draw from it pretty quickly. When it's tucked, however, you have to clear your tucked in shirt and that takes time. I only tuck my shirt in when, socially, I'm expected to. Otherwise, I have an untucked T-Shirt or Polo over it.
For reference, I use a Concealment Solutions Black Mamba (
http://www.concealmentsolutions.com). It's not really advertised as a tuckable, but it can be used as one because of how it's designed. The only issue is the black clips over my belt are visible when tucked. But I usually wear a black belt so the clips just aren't noticed by people. The holster works very well either way, but if you are mostly going to wear a shirt tucked in, get a holster specifically advertised as a tuckable. Several have been mentioned so far.