The Justin Timberlake and Jessica Biel relationship is a rarity amongst celebrities in that it has actually already last a long time. I think they officially got together in 2007 and have been married for quite a few years now which shows they have staying power. If they can manage to be happy for so long already why shouldn't it carry on!
Yes, they are a really nice couple that are in love with each other. They have a bright future ahead of them and they will both work on their relationship to make it work and last because they have a kid in common. She has been very supportive of him over his Golden Globe Awards Nomination for Best Song.
I would love to think that Justin Timberlake and Jessica Biel will last, but there is just too much pressure on celebrity couples. We see them break up over and over again, at a higher rate than people who don't garner all that attention. It's hard to have a healthy relationship when the whole world is watching.
The relationship between Jessica Biel and Justin Timeberlake is unlikely to last. It seems that Hollywood relationships do not last. For that reason it is unlikely that this one will as well. I suspect that one of the biggest factors in the short-lived nature of Hollywood relationships is that the individuals are frequently on the road and away from one another.