I honestly see no reason to get a bipod unless you are using a large rifle. Basically, if it isn't at least as big as an M16 or M14, the bipod just seems like you're just adding some metal to a weapon that doesn't need it. Some carbines such as the M4 and G36K can justify having one, but I'd reserve these mostly for snipers and larger weapons like M249s.
Now, when you talk about purpose, you don't want to go for the $50+ ones, you just need the one that gets the job done, will last a lifetime, and is easy to use in action. Other than the Echo-1 614 I've got, the other AEG I own is an M14, and for steady aiming, a bipod is a must if you can't prop your gun on a rock or something convenient. If you indeed have a sniper rifle, spring, gas or electric, I would certainly recommend this one.
Hopefully nobody is actually this insane, but just for kicks, someone at the last event I attended had mounted one of these on his pistol rail.
Feels nice and strong, but don't run over it with a car or something.
Easily stored and deployed in action.
Cheapest "good" bipod out there (DO NOT go with the bipod-handgrip thing, because I know two people that found that it wore out quickly and the bipod got stuck hanging out, and it's just not that great overall other than its small size)
Completely eliminates the problem of fidgety left hands (or right hands if you're a lefty)
Overkill for a small weapon. For a compact rifle or submachine gun, your money is better spent on a vertical foregrip or red dot scope.
May add some imbalance issues to some rifles, particularly the M16 and other ones that are already front-heavy. But if you're using the bipod, the weight of the front-end won't matter anyway. This is just for if you're not using it.
If it's folded up, it may quickly become a hinderance. I found that it just wasn't worth it on the 614, and it's best to just keep it down all the time unless you absolutely have to fold it. (This only applies to when you're using it. In no way am I saying to leave it unfolded when storing the rifle).
Rating: [5 of 5 Stars!]