These people

eBay link

sold me a 2300 mAh BP-5L compatible battery (I know it says "1700 mAh" on eBay, but 2300 mAh is what I got) for a very reasonable price. I can confirm that it fits and charges normally in my 770 and that its runtime is certainly longer than the standard 1500 mAh battery.

Maybe the specs are indeed optimistic, but for EUR 9.95 incl shipping I'd still call it a good deal.

And before anyone asks: No, I don't have a share in that company.

I'm a bit worried about buying 3party batteries. Can we be certain that it has the built-in protection circuit? There have been enough incidents with batteries with missing protection circuits (everything is fine until the charger circuit in the device fails.. then boom). It's very easy to buy cheap "raw" batteries in bulk and easy to make a product out of them.. very easy, if you skip adding the circuit.

Well, for the same amount I recently got 2 sido batteries, a desktop charger and a retractable usb charging cable. The batteries are neither 1700 or 2300 but they last the same as the original battery.
The charger isn't very sophisticated (3 transistors and a bunch of passive components) but it seems to work (crossing fingers).

My Sido batteries don't last nearly as long as the original in my N800,
and the power jack on the battery charger broke immediately.

Ordered a couple for the ole 770 & 800... unlike many vendors they only charge shipping once, which makes it even better.

That way we'll have something almost like a statistical sample to evaluate in a while :-)

Maybe one of you guys can buy an extra (as they're cheap.. ) and check if it's got the protection circuit. If it doesn't, I wouldn't use them. If they do, they're a bargain.

What does a protection circuit look like, and how does one get at it ? :-)

You have to open the battery (at the connector end) and check if there's a very small circuit board between the actual battery poles and the connectors.
NB: opening a lithium battery is not without risk. Avoid punching a whole into the actual cell walls.

Maybe if someone has access to an X-ray machine can check this without opening the battery...

I'm no expert, but I'd say that the cost of the protection circuit is a tiny fraction of the cost of the cells, so I'd be surprised if they omitted the circuit.
It's much more probable that they're using lower quality cells (which, btw, is what cause explosion of brand name batteries with sony cells), or simply that those batteries don't have the markup for marketing/highly paid execs.