IPhone 3GS on STRAIGHT TALK- YAY OR NAY?

I'm currently a straight talk user, although I know the customer service is terrible the bill is cheaper than my previous carrier- Verizon. When I was with Verizon I had the iPhone 3gs and absolutely loved it. My younger sister has one she got off of eBay, unlocked, and activated it with page plus. NOW; MY QUESTION IS: Does anyone know if the iPhone works well with straight talk? I'm looking into getting one unlocked off eBay as well. But was told that if the original carrier of the iPhone effects the use of the phone once activated with straight talk. Any one familiar with the services?

First, I'll help you with the customer service problem. All I want in exchange for this info is to please tell other straight talk users about it, because it's been so helpful:
Don't call the regular straight talk number. Call the Tracfone Florida headquarters. They own straight talk, are much nicer, a lot more helpful, and clearly speak English. And you will always talk with a real person. The number is 1-800-876-5753.

Now, your original question…
This is sticky. See, the sim cards straight talk used to offer were through at&t. Remember, the BYOP (bring your own phone) plan? You could bring in your unlocked at&t phone and get a sim from straight talk super cheap. But then, at&t got a stick up their but about it, and told straight talk to drop the byop plan. And because straight talk has to balance between what users want and what the companies, that let straight talk use their towers, want. And between us (the users) and the companies, Straight talk will always try to please the company first. So the issue now is:
The sim cards are still available on eBay, but people are selling them VERY expensive now.
If you unlock your at&t phone, there has been several cases where at&t relocked the phone (again, refer back to the 'stick up their but' part.) Now if you got a friend who insists that she unlocked her at&t phone and never had a problem, great for her, sure. But it has happened to people. What's the percentage of chance that it'll happen? I don't know. Is it worth it to you to try?
And those who've used the straight talk service with at&t towers have complained A LOT about data speed, throttling, and a data cap. Now, I have all these issues and mine (Samsung galaxy proclaim) uses Verizon towers. Straight talk won't say exactly what the soft data cap is, but it DOES exist, other straight talk users can vouch that with me (I've had my service shut off twice within two years of owning smartphones through them).
So you see, an unlocked at&t phone with straight talk can definitely run you into some problems (remember, CAN doesn't necessarily mean WILL, but… ). With a Verizon one, you can do something called 'flashing', a fairly complicated process that let's a Verizon phone use straight talk without a sim and requires a 'donor' phone. There's talk about how long this process lasts, whether it's permanent or reverts back at whatever random moment. Something to look into and research before getting someone to do this.