If you're looking for decent medium duty shelving...

[K Frame]

I recommend these from Home Depot. They're on sale right now.

http://www.homedepot.com/p/HDX-55-5-in-H-x-36-in-W-x-18-in-D-4-Shelf-Plastic-Ventilated-Storage-Shelving-Unit-in-Black-140618/300007255

I bought two some months ago for the basement, and I'm getting ready to buy a couple more.

I'm currently using the first two in the back room in the basement for tool storage. I'll use one of the new ones for additional tool storage and the fourth one for firearms related stuff.

They're lightweight but sturdy.

[Hawkmoon]

I don't trust plastic.

[K Frame]

I don't trust plastic.

My Mother bought a couple of sets of plastic bookshelves in the mid 1980s.

As of last week, they're still going strong after close to 35 years, tons of moves, and tons of books and other crap being piled on them.

[Brad Johnson]

Used indoors they should last a good long time. It's high temps and UV that kill them.

Brad

[K Frame]

"It's high temps and UV that kill them."

Ding ding ding!

We kept a similar set on the back porch for stuff like garden tools, etc...

It got full sun during a good part of the day.

They didn't last 10 years before they shattered.

[RocketMan]

I've had four of those exact shelves in my basement man cave for three years now.  They are holding up well.
Unfortunately, they weren't at that low a sale price when I bought them at Home Despot.

[dogmush]

I don't trust plastic.

For $20, in America, shelves come in plastic, or particle board.  You'd be stupid to trust particle board over plastic.  I suspect you'd be hard pressed to find metal or solid wood shelving that size for less than about double that price.  Maybe triple.  You'd be hard pressed to buy the wood and MAKE a solid wood shelf for $20.

At this point in materials science saying "I don't trust plastic" without some kind of use modifier is pretty ignorant. So +5 points for cranky old man cred, but -10 for not actually saying anything useful.  Now get of the lawn.

[Ben]

That looks almost identical to the two that I bought from Costco, I think for ~$5 more each. Mine have held up very well.

I will say that the one I store ammo on is slightly sagging on a couple of middle shelves after a couple of years, but that's on me for overloading it with full ammo cans. I should probably swap that one with metal at some point. The other one holds auto maintenance supplies and some camping supply bins and the shelves are as straight as on the day I bought it.

[HeroHog]

The "Gorilla Rack" plastic shelves can hold a LOT of weight and are very durable! I have a plethora of plastic shelves in our home and garage.