I've got both the Kramer and 511 undershirt holsters.
I prefer the Kramer (Confidant). It has actual "holsters" attached to the shirt and holds things in one spot without a lot of movenemt. The 511 just has pockets the gun slips into, and the guns are not really positioned or held in any one position and tend to move around more. You are limited to smaller guns too (or at least for me you are).
I've worn my Kramer with a Colt Commander on one side, and its double reload in its belt carrier on the other, under a full cut oxford dress shirt with no troubles. The weight of the heavier guns and their gear does start to wear on you as the day goes on, but its still very doable. I do find two SIG P230s are much more comfortable though.
Access isnt all that bad, and really no slower than anything else under a tucked in shirt.
Kramer suggests buying the shirt one size smaller than you usually wear, which I did. This helps keep things snug, and from flopping around. With the lighter guns, its very comfortable for day long wear.
The other advantage to the Kramer over the 511 is, you can wear a normal tee shirt under it, where the 511 "is" the tee shirt. Another issue with the 511 is, they come in "white" (make that brilliant, lit up white!) and dark blue (if I remember correctly). The white shirt makes you look like you just got out of boot camp.
If your looking at the undershirt option, you may also want to look at the Smart Carry. Since I got mine, I dont use my Kramer much anymore. The Smart Carry is much more comfortable and versatile, but it does tend to limit the size of your gun, or at least for me it does, due to my choice of pants. With the right clothes, you can carry pretty much anything. You also can do it in sweatpants or shorts, and you dont even need a shirt.