Well, I don't think he's a bad champ, not at all, but I think you should try it before you buy it. I personally don't like him, but many friends of mine play hecarim and they do a pretty good job, specially jungling.
He is a strange champ... I love him, but some of my friends hate him cause the lack of CC for ganking (his E used in the wrong way could save the enemy)... But overall he is good
Don't purchase him if you never tried it... Ask a friend that owns him to 2-3 games and/or spectate some games with a Hecarim before buying him
Its a nice character ,good team fight iniciator with his ulti(R) as the begining -it fears and deals nice damage :D ,but it also can be used as escape skill - like Shyvana's ulti. He can make quite nice dmg got aoe dmg skill that heals him (W) and a skill that hits everyone around him (Q). He can be used as a chaser with his (E) ability.
Another goodie is that his passive gives you bonus AD depends on your speed - so you need to get some movement increasing gear for aditional AD and Hecarim ignores unit collision its good for chasing/running away .
He can be a nice jungle ,and as an iniciator its good to make him tanky.
If i were you i would buy him :D
he is fun to play and i mean very fun, but if are looking for a very good jungler champ a lot of champ fix that role better than him and are more cheap, (udyr, amumu, alistar, maokay) they even clear the jungle more faster that him and have better gank potential but in the end the final choice is your xD