First of all, let's clarify what we're talking about for those not in the know. Tobiko is the tiny, orange, pearl-like stuff you find on sushi rolls. It's actually flying fish roe, which technically makes it a caviar (albeit less expensive than its sturgeon cousin).
Tobiko adds crunchy texture and salty taste to the dish, not to mention artistic flair. It even comes in multiple colorsblack, orange, red, greenall with different flavors and levels of spice. In its natural state, however, it is not very flavorful. Only after it's been processed with simple preservatives and flavoring does it take on the form most of us would recognize.
As for whether or not it's nutritious, this is a difficult question to answer confidently given how little is known about tobiko processing. Little nutrition information is available for tobiko at this point because it's not a mainstream (no pun intended) food. However, it seems that it is relatively low in calories and presumably a decent source of protein and selenium, a trace mineral important in the production of antioxidants. The short answer: it's not bad for you.