Let's face it, any bike is as good as you make it or want it to be. I have a bunch of Bultaco's, some that I even race on a regular occasion, with success and ultimate dependability. They are as good of a bike as their mechanic's and riders. Take any bike and poorly maintain it and beat it like a rented mule and it will treat you the same way.
It all depends on what you want to get out of the ride. I'm lucky enough to live in a rural area with plenty of riding. If 6-8 friends stop by and want to hit the trails and dirt roads hard, yeah I'm taking my '02 KTM EXC 520. If it's a nice spring morning and there's not much work to do that day, me and my original un-restored '68 Yamaha DT1 is going to cover over 100 miles of dirt roads that day. If I need to get in shape from lack of excersize or need to let off some steam, a couple hours on my Champion MX framed YZ360A Yamaha on a vintage friendly MX track is surely the cure.
You can have fun on all of them. If you are lucky enough to have the wearwithal to have more than one, then you're in. If not and you choose to ride an old bike, so be it. Enjoy it to the utmost, whatever bike it may be.
It also doesn't hurt to have something different at the end of the ride, if you need a conversation starter. I can stop at a local establishment on my KTM and no one takes a second look. Wheel up on a '68 Yamaha and the conversations can go on for hours. What does hurt is staying 'till after dark on the KTM isn't a problem, on the Yami, you want to be home before dark, 'cause there's a good chance of no lights.