Ventajas
NI is a very flat company, which encourages both horizontal and vertical movement. One can easily move from a field sales position to an R&D position, or R&D to marketing, etc, and employees are encouraged to define their own career paths. The company prides itself on job security, having not laid off anyone in its 30 year history except after acquisitions. Because NI features such a broad product line and is involved in so many different industries, it is less prone to the extreme ups and downs of other tech companies.
Desventajas
NI pays about average for the industry, but aims to hire "the best and the brightest." Upper management goes so far as to admit that "if you're out to make as much money as possible, you're at the wrong place." Salaries are only adjusted once a year, even if you are promoted or change positions, so there is little motivation to push yourself. Also, the skills that are developed (for example, LabVIEW programming) are largely tied to NI products, so you will likely find that any future positions you might take (even with other companies) will be tied to NI in some way.