The most important part of owning or modifying any car is that you do what you want with it. After all, it is your car. No car is worth putting money into unless you deem it as such. I've had my car for two years and kept saying it "wasn't worth putting money into" because it wasn't what I wanted. Eventually though, I realized keeping my Protege was the best option as it's reliable and cheap to fix and I can save for a different car but still modify the Protege while I have it, so I've starting putting money in it and I don't regret it. If you're looking for something fast then you're in the wrong place, but if you want a car that gets good gas mileage, cheap to fix, and reliable then you can't go wrong with the Protege. Throw some decent suspension on it with a good set of brakes and tires and they're a blast to drive and well worth the money. Not to mention it's something different. I went to a meet this week with 3000+ cars there and my Protege was the only "base" there (3 MSP's, and 1 MP3).
If you think it's worth putting the money into, then it is; if you don't think it's worth the money, then it's not. Simple as that.