It's pretty good. Not only does it give you the definitions for a GK link in your commentaries, but it also shows you the reference to the TDNT. Although, as noted by "Anthony Grubb" below, it doesn't actually open TDNT, there are some workarounds. It's sad we can't get a free Strong's Concordance with Logos, and that it was necessary to buy this, but that's another issue. For what it is, it's decent.