Merits:
1) It's a beeeg mufta! Nothing takes care of you like a Fulbright scholarship!
2) You actually become a part of the Pakistan Fulbright community which is a fairly big benefit on top of the mufta. Other Fulbrighters become your support system in US/on Facebook, your hosts all over the US and finally can potentially help in getting you a job when you come back.
3) It's pretty damn prestigious! Throw that in your introduction in the US and you'll be a celebrity. Throw that in your job interview once you come back, you'll definitely earn the interviewer's respect even if you don't get the job.
De-merits:
1) You gotta come back right away! Once you are there and organizations are coming to hire other graduates, one does think about the lost opportunities. However the prestige is worth it probably to offset that in terms of getting you a job once you're back. But do keep in mind that getting a job isn't exactly a walk in the park either. Conclusion: Think about your life goals really hard before accepting it (not before applying, you might just not get it and otherwise will keep regretting "oh darn! I could have gotten a Fulbright scholarship, when all these nincompoops got it!"). If you really want a US Masters/PhD and you can't pay for it otherwise, and you won't complain all the time while studying there and when back, "see, this is so sad, I have to go back right away!" then definitely go for it and take it. Otherwise, please look at other means to finance your masters/phd or don't accept it at all.
Thank you!