Involvement:Expert (understands the inner workings) - Ownership:more than 1 month

-Cheap!
-In the right dress and in the right mood, they really make a difference.

-Actually pretty heavy. Be prepared to experience neck or shoulder pains if you don't own a good bra that can support the extra weight.
-Pretty big. Girls with flatter chests can't pull these off without it looking weird and bras with thinner cups will show weird bumps.
-Not very comfortable if you're in temperate areas and/or sweat somewhat. The material feels icky and develops an uncomfortable smell and starts itching after a while (10 to 15 hours in :D)
-If wearing a very restrictive bra, these will press on your chest pretty hard, since they're not actually all that soft. They will feel harder than natural breasts.

-Smaller, low-cut bras will not hide the edges of the inserts completely, so be careful with what you do with your new, improved cleavage!
-They dont stick to your skin, so one unfortunate bending over and your insert might plop out and down onto the floor.
-These take some amount of natural breast to work. Larger side of A works with the right bra, but a small B and upwards might be best.
-I doubt male-to-female performers could use these because of this.
-For prolonged exposure it might be nice to encase these in something that sucks up moisture.

If you're looking for more ooomph in your chest area for a party or a date, clubbing or other special occasion, go for it! Easy to insert, easy to remove.