While the whole metallic to ammonia issue may exist, let's face it girls... $60 (as a minimum!) vs. $10 (or less!) is a huge difference. And I'll be honest with you, you'll notice more of a difference and be unhappier, I think, if you go somewhere to color your hair that is not as nice as you are used to... rather than just save the money and do it yourself. You won't be expecting perfection for $10 in your own bathroom so you're more likely to be satisfied with it... unless you are a light to platinum blonde. If this is the case, I wouldn't try to do it myself. It's almost impossible to keep brassy tones out of your hair with a box DIY kit if you want light blonde tones.
However, if you are looking for a dark blonde, red, brunette or black, go for it! It's easy, affordable and totally fixable if you screw it up. Today's hair color kits have very specific instructions, are user friendly, include everything you could possibly need and I love them! (You can even do high lights and/or low lights!) I've been coloring my hair for 17 years, have had some bad experiences and with a few teenaged "expressing myself" years, have always gotten good results, compliments and felt good about it.
I'm a big fan of Loreal Feria and for years used Nice N Easy. I like the conditioner that comes with Feria and my hair never feels nasty and dried out afterwards. There are a lot of shade options, depending on where you buy it (Don't settle.. if you don't see what you are looking for, look at another store!) I do recommend, like other posters, that you absolutely do the test... especially since you are a first-timer. Make sure you like the color you see before trying it on your whole head... and that you are not allergic!
And get a girlfriend to help you! It can be tiring, (especially if you have a lot of hair!) to get the back. Don't be afraid to use your plastic combs, etc to make sure you get the color all the way through. Coat your hairline skin with vaseline to protect it from staining and wear a button-up junk shirt you can easily shed. Use dark colored towels to dry your hair after rinsing and make sure you clean up any splatter right away! (Have a wet washcloth ready in case you need it for clean-up! Dyes can stain your countertop, sink, floor and/or walls.) RINSE RINSE RINSE!
Don't be afraid! You can do it! It's fun and mostly fool-proof these days... Good luck and keep us updated!