The pros and cons of singed are simple.
Pros:
Great farmer
Great lane presence all game with this build
Great pusher
Good in team-fights
Tanky
Good dps with this build
AOE damage
Epic toss that can ruin someones day
Can overextend
Can win against many usual top lane champions
Dominates melee champions
Has no fear
EPIC LAUGH

Cons:
Sort of weak early game (without this build)
Runes and masteries are very essential to a good singed build
Can be kited in some situations
Can be beaten by ranged champions
Has a difficult time very late game if champions on his team aren't strong enough to fill his falling damage levels
High skill to master, requires knowing that you will survive with low amounts of health after a kill in many situations, you must know your tank extremely well at all levels.
Must be played aggressively to get the advantage, can lead to being ganked in some situations.
Basically has a big GANK ME over his head at all times.

Singed is a great champion, he has great mobility, good damage, strong tank, and his ultimate is incredible for towerdiving, escaping, and damage.

The reason I take these runes and masteries is simple, most people don't take these and go more of a tanky item build this is because they play singed primarily as a tanky dps, they focus on tossing people into their team, and trying to get attention, usually this isn't as effective as my build for a couple of reasons.

First off Singeds weakest points in the game are any time before he has his catalyst, and any time after he has all 6 item slots filled because after that point he starts to become less and less effective. Therefore my runes and masteries directly affect the most important part of the game, before he has his catalyst, he has ap, armor, and ap/lvl to help a bit at the end when he starts to become less effective along with the obvious magic pen reds. Most people would laugh at this say, no movement speed quints? ha ha ha, but I have ghost, thats more than enough with my ulti to have mach speed movement and separately they keep me moving quickly for just about any encounter.

The other thing people may laugh at is ignite, ignite is super important to finish kills on singed, he is a great damage dealer early game with this setup, he pushes his opponents around and then he can finish them which most opponents wont expect from a singed.

These items also look a bit different than most peoples impressions of what to get on a singed. Most people go 1 ap item then one tank item. The reason I get Rod of ages and rylias before anything else is simple, singed is an incredible farmer, the more ap you have, the better you farm, the more you can push your opponent around in lane, and the less people can chase you at low hp. This helps a good singed in every way, he is unable to be chased without consequences, he is still really tanky due to masteries and the fact that both items give hp as well as ap, and hes just more of a disturbing laning presence in general. I find that once I have rod and the ap items of rylias teams will send 3 people top to deal with me, and then I just laugh at them while ulti or ghosting through whichever one of them has the least cc, or leaving as soon as mid leaves their post.

You can end the build with a deathcap, void staff, or any tank item depending on what you need.

One of the main reasons I play singed is, short of throwing the whole team of you there's very little they can do about you pushing top lane, a jungler gank will often end up in a double kill for a good singed, meaning they often take mid off the line to deal with you, going 1 for 1 as singed is advantageous due to the fact that he is incredibly good with money mid game, also if he is returning to lane at the same time as the opponent hes right back to denying the **** out of them. I overextend with singed, I personally think that he is the best champion in the game at overextending as if it doesn't matter to him if they gank, great dps, great tank, great escape.

When you see the enemy team having a tryndamere and a jungler, or a gp and a tryndamere you should lock that singed and laugh, you pretty much just won the lane.
Singed has a hard time losing to anyone if played right, the worst thing that can happen is like a cait going top solo. you cant close the gap, she can kite all day.
Tryndamere has been claimed to have a better early game vs singed I disagree with this build I have yet to lose to a Tryndamere, also I do not get cloth armor like many would suggest, I feel my dps is much higher and kiting him is simple.
Basically its simple
Ad Melee < I win (Optimal)
Ap Melee < Can tie and win in some situations (chos annoying)
ad ranged < YIKES hard to even tie, can win if stupid, beware kiting.
ap ranged < not quite as bad but still not that great. Can tie, can even win if lucky