There's a helpful post somewhere in the forum to make really good bubbles, but I lost the link. Basically, you use the Shape mode for ovals, and then you use the Path Selection tool (black and white arrow in the last section, should be hidden in the list under the hand icon) to move around the corners of the bubble and adjust it around the text, and even click the points to bring up arrows to adjust their width and angles. I use this with a white oval and later I use a layer setting to have black borders, but it should work with just lines too, I think.