[Lady Of Light]

It seems to me that Gollum/Smeagol had a weaker soul than most. And he started as a hobbit-like creature, while Bilbo, Sam, and Frodo held up quite well against the ring's power (but for Frodo at the very end, but that was totally inevitable.) Gollum had the ring for hundreds of years, longer than any of the four, and that allowed it's power to take complete control over him. It became an obsession, a never-ending search for his "precious". Gollum was tied to the ring probably almost as much as the ring-wraiths. If he had not been thrown into the Cracks of Doom with the ring, I believe that he would have died when it was destroyed. It was all that he lived for, and though he faced starvation and death, he kept up his search. It was not so much food that sustained him, it was the ring.
Bilbo only had it for sixty-one years, a comparatively small amount of time. And while he did not wish to give up the ring, he realized what was best for the world. He was obviously made of stronger stuff than Gollum, who killed his friend at the very sight of the ring. He began to age faster immediately because he did not care for the ring as Gollum did, he let it go.
Sam did not have a completely easy time with the lust of the ring. He even saw himself for a moment as a great shining warrior, using the ring as the source of his power. He liked that image very much, but he came back to earth and realized that Frodo was the ring-bearer, and that he could never use the ring for good. Yay for Sam!