Here's a direct comparison showing what one might expect from a Raynox close-up lens. I took photos using a good "real" macro lens with and without a Raynox DCR 150: (overview is a slightly reduced upper left quarter of the full photo, the cursor arrow was in the LCD photo's center)
Both the center and corners are sharp and square.
Now, with the Raynox (mag slightly lower)
Center and corners square,but corners lose sharpness.
The Raynox does a good job if center quality is sufficient for your composition. Raynox is pretty good if you crop the edges bit.
An Iowa Dave
PS A square region of the Raynox' image as tall as the frame is in good focus; ie. about 2/3 of the width and full height is good. One might easily compose with that in mind.