8x are OK, but I prefer 10x. The second number refers to the objective lens size. The larger the objective the more light gathering and therefore the clearer the image, generally speaking. I say that because the quality of the glass and the coatings can make a big difference. I think a set of 8x21 are going to be somewhat borderline, depending on the build quality. That said, I have a set of Pentax 8x21's that I love for a small packable set that I keep in a car bag. They were brighter and clearer than a similar set of 10x25's and I was able to read signs with the 8x21 that I couldn't with the 10x25's despite being lower power. A good pair of 8x will be better than lower quality 10x. My primary set of binos are Nikon 10x42's. I also like a 10x42 monocular I have that sports a mil-dot reticle made by Bushnell. The Nikon's are better glass, but the Bushnell's serve a different purpose. I can't afford the really high priced glass, that I'm sure some here have.
As power goes up the objective needs to be larger to maintain brightness. 8x21's are usually pocket bino's. You should look at 8x32 or 10x42 as a guideline for binos you're really going to use vs sightseeing. Remember, you may also be using them at night.
You should go to one of the big stores, Cabelas, Bass Pro, Academy, etc. and compare them so you have an idea of the differences. I usually test by looking into the shadow areas such as up into the rafters across the store. Can you see more detail with one over the other? Reading signs across the store works well too. Everything looks great in bright light. Look for shadows. Another thing I like about the big box stores is that you get to see what stuff looks like after being (mis)handled a lot.
Warning: Once you've looked through good glass, it's hard go back.
That said, FREE? Where's that rebate? I could use some for the kids.
ETA - Just looked at the 8x21's. Note they are not waterproof/fog proof. A factor to consider if you're considering field use.