I think the effect is to corrupt slowly, but the actual effects will depend on the person/being who owns it. As time goes on, it seems that the hobbits realise the effect quite early on, and tend to resist the changes - Bilbo and Frodo both show this. Of course, with continued use/ownership, the corruption becomes more and more invasive.
In the case of Gollum/Smeagol, I think that the corruption affected both of them, but the psychological strain of resisting broke his mind, and gave the split personality. One personality succums more than the other, but they are both corrupted.
If Frodo or Bilbo had kept the ring for a lot longer, they would also have had their mind broken, although the precise effects would have depended on their own weaknesses. From the evidence, I suspect Bilbo would have become grumpy, possessive, paranoid. He would have been able to put on a pleasant public persona, but underneath that, he would have been very miserable. And this would have isolated him from everyone.