Well, I have seen a lot of people play Ezreal and fail so I decided to shed some light on how to make him useful. This is an AD secondary carry build.
The first thing people have to realize is that Ezreal CANNOT carry. Reguardless of whether or not he goes AP or AD, he just doesn't do enough damage to be the carry. His main job is to be the secondary dps hero and use his w to help the main dps whenever posible. Therefore, I rarely go middle if there is a solid ranged dps or nuker on my team. Ezreal can hold his own mid and he can dominate against weaker players but if he ever runs into a good team, he will fail. That being said, only go mid if your team doesn't have a good solo hero or if you think your team is going to suck.
I find exhaust/ignite to work well for Ezreal. He really doesn't need flash (because you aren't going in before your tank right?) and the exhaust/ignite combo is very good for getting an early kill or two. However, don't be afraid to swap ignite for either rally or clairvoyence if your team is missing either of those skills. If it looks like your team isn't going to have any dps besides you, then you should get cleanse instead of ignite so that you can stay in the fight longer.
Runes and Masteries
I go armor pen on red, dodge on yellow, MP/5 on blue, and HP on quints. However, I'm going to assume that most of you don't have 20K worth of IP to spare so just use whatever runepage you usually use for a physical attacker. As for masteries, I usually run 21/0/9 because I run exhaust/ignite but you can really do whatever.
I usually get a level of each skill (e first) and then r > q > w > e. Q is good for harassing and last-hitting while you are laning so that is maxed first. Since this is mainly an AD build, e is only used to escape so it is saved for the very end.
Long sword + health potion
Boots of speed
Sword of the occult
Stark's fervor (if your team doesn't have)
If the game is still going on or if your team already has a starks, get a black cleaver and then either banshee viel or guardian angel depending on what you think will make you survive longer.
*edit: this item builds assumes you have Mana/5 for the blue runes. I haven't tried Ezreal without my Mana/5 runes but I assume he will probably need a chalice or some sort of mana regen before frozen mallet. I'm open to suggestions as far as that goes.
-STAY IN THE BACK. Seriously, you should never be next to your tank. Take advantage of the range on your q and spam that during a battle.
-Use your W spell on your carry. I see many Ezreal trying to use this skill as a nuke. Don't be one of those people. It isn't worth it to run right up to the enemy team and try to hit all of them with the AS debuff, you are more usefull to your team just sitting in the back and buffing your carry
-Use your ult earlier rather than later. While it is tempting to use your ult to finish off running heroes, it is usually better to use it at the start of the battle. You will do a good chunk of damage to the other team and cause 1-2 cowards to run back giving your team a temporary numbers advantage.