Pros
NI works very hard to provide an awesome work environment for its people. They also strive to be a reliable employer. They put their employees first. When other employers lay off in droves, NI hangs on, relying on cash they have consistently put away for the inevitable economic recession. The culture is very easygoing. The people and management are excellent. It's technically challenging (at least in R&D) but not overly demanding. Some companies say they encourage work-life balance, but NI lives it.
Cons
NI salaries are not the highest, although this is somewhat intentional since NI doesn't want to attract salary chasers who change jobs every two years. They also don't have some of the nice perks that some fast-growing tech companies have (e.g., free cafeteria, free fitness center, etc.).