Here are some advantages and disadvantages to buying an early '80s Japanese bike in general, with a little content specific to the Nighthawk. For the record, one of my current bikes is an '83 Yamaha Seca 550 (though there is a Nighthawk down the street from my house that I've been eyeing...)
BRAKES: My Seca has a single disk up front and drum in the rear. I believe the 650 'Hawk has dual disks up front and drum in the rear. In either case, compared with more modern bikes, "adequate" is a generous description of the stopping power and lever feel of these setups.
SUSPENSION: The swingarm will feel like a wet noodle if you hit rough pavement in a tight turn. The forks will dive hard when you grab a fistfull of those wooden front brakes--upgrading to progressive springs should be a first priority. The dual shock setup in the rear is fine, though the seals are likely to be weepy and the bushings shot if they're still the original units.
ELECTRICALS: Time and exposure to the elements does bad things to wiring. Gremlins have likely taken up residence somwhere in the harness. If so, you're going to have to chase them out with a multimeter and a soldering iron.
UNKNOWN SERVICE HISTORY: Did a prior owner run the bike under the mistaken assumption that the motor oil was a total-loss system? Do the wheel bearings sound "crunchy"? Does the inside of the fuel tank look like a postcard of the Painted Hills National Monument? There's just no knowing what amount of abuse and neglect this bike has been subjected to in 20+ years.
AESTHETICS: I really can't put into words how much I hate square headlamps on bikes.
IT'S CHEAP: At least, it ought to be. If you're paying more than $1500 for this bike, it's either in showroom condition or you're using Canadian dollars.
IT'S SIMPLE: A shaft drive in good condition is no-muss, no-fuss. Carb tuning, though viewed with horror and suspicion by a lot of riders nowadays, is no black art. Yes, synching four CV carbs is a bit of a PITA the first time you do it, but you'll get the hang of it quickly and it doesn't need to be done very often. Plus, being good at it (and owning your own Morgan CarbTune) gives you a kind of instant celebrity status in crusty-biker circles. Oil changes don't require 45 minutes of plastic body-panel removal. The list goes on and on...
YOU CAN TINKER: Bikes like this aren't so old that you can't get parts for them anymore, but they are old enought that you can always justify swapping out this or that part by saying "it's worn out". Break something? Go to the wrecking yard and get another of the same. These are great bikes to learn maintenance on.
IT'S A STANDARD: More than any other, perhaps, the Nighthawk is the quintessential UJM. It's a little bit sport bike, it's a little bit touring bike, it's a little bit commuter, it's a little bit everything.
IT'S A HONDA: This goes a long way towards mitigating the unknown service history listed above. Hondas of this era are well-known for their reliability in the face of ridiculous abuse.
IT COMES WITH A SUPPORT GROUP: Ain't the internet grand? Just about every bike has one or more websites, user groups, email lists, and owner clubs. Check out Micapeak, Usenet, and Yahoo to find groups of like-minded Nighthawk owners who eat, sleep, and live all things 'Hawk-related.
So, final analysis: A modern 650 (like the Suzuki SV650) will ride circles around an '84 Nighthawk. Mechanically, it is one or two generations more advanced than the 'Hawk. But what do you expect? It's brand-friggin'-new! On the other hand, it will also set you back at least another five grand over what the Nighthawk will cost you. And, being new, it doesn't invite the rider to delve into the delights of shadetree wrenching. From the standpoint of "smiles-per-dollar", the Nighthawk is the better value.