I don't like the appearance of bra straps with tops that allow them to show such as tank tops or dresses with spaghetti straps or no straps at at all. I know it seems to be a fashion statement in the <20 year old set but I don't personally think it looks good on GG or TG women. Bad fashion sense.
Strapless bra solves this problem. Generally the band at a given size will feel tighter than you might be accustomed to as the bra is designed to have the band provide 100% of the support. Many have "bump pads" in the lower portion of the cup to give a push-up effect which works well with many of the fashions you'd be inclined to wear with a strapless bra. There are numerous other variants including longline and bustier types, which are commonly worn under special occasion dresses such as wedding dressed and formal wear to provide additional smoothing and shaping. Gednerally, if you are large, tend to sag, tend to need a bra for proper shaping or for breast support in pursuit of comfort, this is your choice. Don't let the shoulder straps show!
The silicone adhesive bra is an interesting concept and has quite a following. Amongst CDers, it is often used to provide cleavage when there is very little breast tissue. Search the web, in particular YouTube as there are numerous videos instructing the use of these devices for this purpose.
For the most part, this is a device to use in the most revealing fashions where even a conventional strapless bra would be a problem becoming visible because of the skimpiness of the dress of top. Be aware that this device offers little support so if you need a good bra for support or tend to sag or droop, this is not for you.
I have used the NuBra Light with a fabric outer surface which is a more substantial version of earlier bare silicone bras. This one comes in a regular cup or push-up version in cups A-E. The cup size is mostly a function of the diameter of the cup and to a lesser extent the depth or volume as the cup is not designed to fully encircle and contain the breast tissue. In my opinion, the push-up variant is simply a version with thicker cups but does not provide true push-up effect since there is no band and actual support of the breast. The surface of the silicone is sticky making the cup self adhering to the skin, each cup is applied then the two are buckled together in the center with a small clasp similar to a front close bra. Be aware that this device will not give much support but will accentuate cleavage and add some degree of shaping. It will prevent visible erect nipple "headlights."
Hope this is helpful.
Steph