I had Line-X put in my 05 Jeep within the first week of buying it. I couldn't be more pleased.
Sound: I can not tell the difference between sound of my Jeep with carpet and my Jeep with Line-X. It is at least as quiet, if not quieter.
Heat: I have noticed no problems with heat on the floorboards with Line-X. It is only warm to the touch after long interstate drives. The physical properties of the polyurea in Line-X allow it to tolerate very high temperatures, up to over 300 degrees.
Cost: Every Line-X dealer sets their own prices. Check around. I paid $475 but had to strip the Jeep myself (seats, console, seat belts, seat mounts, etc.)
Other Issues you aren't thinking of: Aside from being a quiet, tough, great looking liner; it's easy to clean with a waterhose if you pull your drain plugs.
If you can part with the cash, look no further than Line-X. The do-it-yourself liners can look good if you really take your time and put some elbow grease into the job, but they simply aren't on the same level of durability. As with everything else in life, you get what you pay for.
[I]A computer lets you make more mistakes faster than any invention in human history - with the possible exceptions of handguns and tequila.[/I]