Is the spring in Tuck Everlasting bad or good and why?

The spring is a bad thing because no one should have to live forever.

On balance, the spring is more dangerous than it's helpful.  When the Tucks first drank from the spring, they had no idea what it did.  They were just a thirsty family traveling through the woods that stopped for water.  Unfortunately, they soon realized that the spring granted them immortality, and that was problematic.

The biggest problem with the spring is that most people will drink from it having no idea what it does.  In this way, they do not get the choice.  It is a very grave decision to choose to live forever.  While it sounds great at first, there are serious downsides.  This is why the family kidnaps Winnie when she sees Jessie drinking from the spring.  If she had taken a drink, the choice would be made before she had enough information to make it.

May Tuck explains to Winnie the dangers of the spring.

"And we figured it'd be very bad if everyone knowed about that spring," said Mae. "We begun to see what it would mean. … That water—it stops you right where you are. If you'd had a drink of it today, you'd stay a little girl forever. You'd never grow up, not ever." (Ch. 7)

Being immortal makes the Tucks outcasts from society.  Because they cannot die, they do not age.  They can’t make friends, or marry and have children.  The friends, spouses, and kids age while they stay the same.  This means that Miles Tuck’s marriage was a disaster.  His kids soon outgrew him.

"I was married. I had two children. But, from the look of me, I was still twenty-two. My wife, she finally made up her mind I'd sold my soul to the Devil. She left me. She went away and she took the children with her." (Ch. 7)

Miles’s case is sad, but at least he had a wife and family once.  Jesse never even tried, knowing that the same thing would happen to him.  He asked Winnie to drink the water when they were the same age, but had she done so she would have been making the choice to condemn herself to the same wandering life as the Tucks.

The Man in the Yellow Suit wants to find the spring and sell it.  People who bought it would know they were making the choice to be immortal, but would likely not be aware of the ramifications.  Living forever sounds great, and most people do not see a downside.  The Tucks do, and they try to protect everyone else from the same fate.