I am fairly new to motorcycling. I have put 7900 miles on my first bike, a GS500F. I will be happy to tell you its positives and negatives. I suspect that most of my comments would apply to the Ninja too.
To me, the best thing about the GS500F is its powerplant. It definetely has enough power for highway travel, and that is important to me. But it is still very controllable.
With a little 250, the top speed is about 65mph. So to keep up with traffic, you must push the bike to its absolute limit. My bike has a top speed of about 100, so cruising at 70mph is no problem.
Some people start with faster bikes but I do not think that that is wise. When you first start riding, you can barely control the damn thing. It is easy to either stall the bike or accelerate too hard. I did not feel comfortable riding until I had ridden about 2000 miles. But I came through that 2000 miles uninjured, and one reason is that I started on an easy bike to ride.
The GS500F also has several handy features for everyday riding. It has a massive fuel tank...over five galons. Since it gets about 50mpg, you can EASILY go 200 miles between fill-ups. That means more time enjoying the ride, and much less time looking for a gas station. Remember that many bikes only have a range of 120 miles. Also the GS500F has a centerstand. The centerstand is a godsend in many situations. It makes the gas tank easier to fill, and it greatly simplifies chain maintenance. Finally, it is an extremely common bike, which helps with maintenance. If you do your own wrenching, you can find MANY manuals to help. There is also a great website, gstwin.com. If you need a mechanic, you can go to any suzuki dealership. I found an excellent dealership that only charges $110 dollars for each maintenance cycle. On European bikes, it is common to pay $250 on each visit.
I really like the looks of my bike. It looks very fast, with a full fairing and a gixxer-style triangular headlamp. I get alot of compliments on the looks of it.
Unfortunately there are some negatives too.
It is very crudely constructed. You can tell that they used the cheapest materials everywhere. The bolts tend to come loose over time, and I used locktite to stop that. It is not perfectly reliable either. I had one oil leak...the mechanic repaired it, but I was off the road for a few days.
Unlike most motor vehicles, it does not come with a modern, maintenance-free battery. You must add acid and water periodically. I live in the snowbelt, and I must garage the bike in the winter. I use a trickle charger on the battery in winter. I have learned to add water/acid just before I put it on the charger, and just after I take it off of the charger in March.
Like all carburated bikes, it needs a substantial warm-up time in cold weather. I ride in temperatures as low as 35 degrees. Under those conditions, it takes three full three minutes to warmup. On summer days it only takes about one minute to warm up.
Oh, I should also mention the brakes. They are adequate, but not great. I have found that you need to pull firmly on the front brake to stop the bike. So it is not like a gixxer, where (I am told) you stop it with one finger. However, once I started to pull firmly on the front brake, I found some real stopping power.
So considering everything, I have no regrets. This bike has been a great introduction to motorcycling. But I do plan to upgrade someday...maybe in another year. Frankly I am looking at a Honda 599 for my second bike. It has more power than I could handle now, but in a few thousand miles I will be up to that level. And the 599 looks like it has better quality than my bike.