Contemplating on buying them, but I might move to the EVO sometime in june. Decisions, decisions, decisions :T
After I got a spare for my OV, I always try and have a second battery for spare, it's nice and handy. It's especially nice if one suddenly dies, you aren't forced to rush out and pay an insane price locally.
I'm going to switch to Evo V as well (I already have the screen protectors

), and as soon as I have it in hand, I'm ordering an Anker for it's primary battery and keeping the stock as a backup. I can handle going a short time with only one, but I prefer having a spare battery. It has saved me a lot of headaches while on jobs that run long. I hate thick batteries/phones and I can't move around an office if I have it plugged in because the battery is dead.
Is the difference between the Anker and the regular battery significant?
If so, then 2 batteries plus a charger for $20 is an incredible deal.
If not, it's still a pretty awesome buy.
It will probably give you about:
10% over a brand new, stock battery.
15%+ over an older stock battery, and tons over a worn out one.
30% over a brand new, generic/junk 1500mah Chinese battery, these are always over-rated and will wear out faster than normal.
10% may not seem like a lot, but it can make a difference in how you get through the day and it still fits inside as opposed to a bulky external protrusion.
Look at it this way, you are going to spend a similar amount no matter what direction you go, unless you buy generic, so why not spend an extra buck or two and get a little extra capacity at the same time. Whether you need it or not, it's nice to have.
Downsides?
You can't get two for the price of one?