i bought olaf a little while ago and started playing him to give him a shot after getting beast-mode'd by him in a few games in a row. i have discovered that he's bloody awesome and fun and if you think about what you're doing with him you can beat almost any top. the ones i've had trouble with were zyra (shaco fed her both buffs early and she was bursting a third of my hp off every time her CDs were up) and lee sin, who is tough anyways. i have had one terrible game with him, a few that weren't too great, and several that were really good or just plain awesome. picked up a quad in a 2v4 running late into a teamfight that had just ended, which was great.
besides the fact that his axe allows him to farm effectively at a long range when against a higher-level foe, he's just plain fun to play. i've been using one basic build with him and modifying it a bit each time, but i'm getting the hang of what's good and what's not on him, and he's just entertaining to play. maybe it's because i haven't played top in a while.
i wasn't as impressed with him when i jungled him. he vomits damage early so it's easy to cream jungle creeps and gank hard when the lane is pushed, but he can't manufacture kills like characters with a solid stun or CTG can (mummy, noc, xin, lee sin). the only reason i'd jungle him is if we had an ap-heavy team - like, an ad/lux bot with a bursty mage mid - to allow a non-jungling tank to farm top.
most of the builds i see for him have him building items like maw, mallet, warmogs, tenacity boots, GA, atmas, and wriggles. i like that set but i feel he doesn't do much at full hp because he doesn't attack fast enough until he's absorbed some damage, and if i'm building mallet and maw before warmogs or atmas, it's tough for the game to go long enough to buy them, even with a lot of farm (you're looking at 10k just for wriggles, boots, mallet, and maw). he's very item-dependent late-game but he's still fun =)