In all likelyhood if its intermittently not reading some disks and flat out refusing others then you might get it working by just cleaning the laser first.
You can do this with a cleaning disk like below:
Hama DVD Laser Cleaning Disc: Amazon.co.uk: Electronics & Photo
In fairness though if your laser is dirty then the rest of your console is probably the same so it would be good practice to open it up and blow out all the dust from both the console and your DVD drive. There are plenty of tutorials on the internet for opening the console up anyway. While you have the drive open you can then clean the laser gently with a cotton bud and some alcohol.
The guy who's disk isn't reading at all should try the same process first then if that doesn't work replacing the laser isn't that hard in all honesty. All you have to watch for when replacing a laser is do not damage the ribbon cable and check if the new laser you're getting has a anti static point soldered as that would need to be removed before fitting.
Thought I might add I fixed my friends Elite recently which had gradually stopped reading disks by just cleaning out the console and laser, sometimes the laser does survive being dirty. Be aware that if you continue using the drive though while it's intermittently reading then the laser is using more power to cut through the dirt on the lens so eventually it will burn out if not sorted.