It's good and easy to get, but has a few bad downsides... First, it offers no direct path to immigration -- it's there for your to do the work and leave when you're done. That's fine for temporary jobs, but not a good way to grow your company. There is no real time limit, however if it looks like you've used it to permanently move to US, the renewals could be denied.
The second is that because it offers no path to green card, the spouses are never allowed to work (unless they get their own TN visas independently), which is not very attractive to families.
Because of these issues, many TN holders apply and transition to H1-B while working in US.