Help identifying bike model and worth

That's an awesome bike - would be called a MTB back in the day, but now we'd call it a rigid MTB.

You probably have to replace the brake pads (the rubber blocks, 4 in total) because they go hard with age/ozone/UV and don't brake as well. That style of brake is called a cantilever and while less-common now, they can still work perfectly well when tweaked correctly. If you did replace the brakes, then your options are more canti's, or V brakes. It may take long-reach caliper brakes, but those generally have lower performance than your existing ones. You cannot have effective safe disk brakes on that bike as-is.

Gears - they likely need a clean, lubricate, and then adjust the cable tension. I see friction shifters on your handlebars, which may feel vague compared to modern trigger shifters. However there's nothing wrong with them.

The cassette appears to be a 7 speed, so it could be a cassette or a freewheel. If you wanted, it could potentially go to 8/9/10 speed, but that would need a new wheel, rear derailleur, cassette, chain, and right-hand shifter.

There's nothing wrong with leaving the front mech as a friction shifter if the rear was updated.

If the tyres show cracks or perishing, then you might want to consider new tyres. For normal street riding, aim to get less~no tread blocks which will allow you to go faster. If the current tubes hold air fine, then just ride it and get some wear out of the existing tyres, and delay spending on replacements for now. The current tyres are relatively street-oriented.

Finally - I really like that paint job. Its a great looking bike and sure beats walking. And since you can't get anything else right now, any bike is better than no-bike.

If your Nishiki Meridian:

1. fits you
2. meets your riding needs

then it is worth getting into riding condition - especially if you do the work yourself.
Air the tires, do a safety check, and ride it around enough to get a feel for how it rides. If you like it then it's worth fixing. If you don't like it someone else will.

Type of riding
It's categorized as a hybrid - a mix of a road bike and a mountain bike.
This means that it rides on the road better than a mountain bike and it rides on the trail better than a road bike.
It also means that it's not quite as good on the road as a road bike and it's not quite as good on trails as a mountain bike.
In short - it's good for general riding. If this sounds like the kind of riding you will be doing you are set. If not a different bike would be more appropriate.

Build quality
Yours is a durable Taiwan made frame with solid middle of the line components that were made to be durable and function well.

Your bike was made between 1989 and 2001
According to Wikipedia

From 1989 through 2001, Derby International marketed bikes in the United States under the Nishiki as well as Univega and Raleigh brand names. Some of the all terrain bikes and mountain bike models were designed in partnership with famed mountain bike designer and Mountain Bike Hall of Fame member R. Cunningham and have his name on the frames. These Nishiki models, though manufactured outside Japan (e.g., in Taiwan, by Giant Bicycles and possibly in Italy by Colnago, Olmo or Viner) often carried the name Nashiki and some of the same model names as had been used on the Kuwahara-built bicycles.[7] The brand name Nishiki was retired by Derby in 2001 in North America.[8] As of 2010, Nishiki-branded bicycles, manufactured by Accell Group were available for sale again in the U.S. at Dick's Sporting Goods. Dick's had obtained licensing rights to the Nishiki Bike brand in the U.S.