I believe that it is mostly institutions that are using the multi-sig wallets.  I know very little about them, but I believe they have the feature where, for example, TWO out of the three users "listed" must sign off on a transaction.  My *guess* is that having two or more sign a transaction would mean more accountability.  Apparently some of the mining pools use multi-sig wallets.

I would appreciate any other basic insights into these 3xxxxx... wallets from people who use them (as well, of course, as to any advantages for non-institutional users).

For me, a multi-sig address will allow the fund in the address to be accessible when, for example one of the holders lost the key.

In this case, the fund can be transferred to a new address with new keys to be distributed to all holders again. More like a fail-safe approach.

A 75% fee discount on signature space when we wake up one morning to see that Seg Wit Soft Fork has been approved by 4 Chinese/1 Georgian dude, and 80% of the node network has entered part zombie status.

The only other reason I can think of is if you want to use the wallet for some type of escrow service where you need at least 2 private keys to access the funds. This would ensure that the transaction is more secure and that any dispute can be resolved effectively.

If you want someone to co-sign your transactions for some or other reason, you use multi-sig ... I on the other hand, do not want to make it too complex, because this adds various extra variables to the scenario, if you
go this route. Questions on availability of co-signers ect, and if something happens to them, will always be a issue. You will find that the advantage of doing it in this way and the added security it brings, might just not
out weigh the troubles you might have in future, if something goes wrong.

Yes, you can essentially make your wallet 2fa without risk that the service provider can run with your coins.

the 1st set of keys is with your hot wallet
the 2nd set of keys is with the service (e.g. copay)
the 3rd set of keys is in your (offline) secure backup

Any 2 can sign. So if your hot wallet is compromised, the attacker would also need your phone (for one key from the 2nd set) or your secure backup (for one key from the 3rd set). If the service is no longer availalbe you can move the funds yourself with your hot wallet and your secure backup.

I don't think so, As far as I know these are most appropriate for Services like middlemen/escrow, Wallets to ensure security of deposited funds and companies etc... I don't think it's useful in any specific way for a person unless it's to control a gambling addiction or some rather different situation.

I think that we all will be using multisig eventually.

Multisig is needed for payment channels, which currently seems to be the hot idea for making bitcoin scale.

The basic idea is that instead of broadcasting every transaction to the blockchain, one party will partially sign a transaction from a 2 of 2 multisig output which the other party keeps as security (knowing that they can sign it and get the coins if needed). Subsequently many more new transactions can be signed by one of the parties and stored by the other, representing a kind of floating balance between the two parties without ever broadcasting to the blockchain. Eventually a transaction is broadcast - settling up and closing the channel.

There are more details…

See https://www.youtube.com/watch?v=8zVzw912wPo

I dont think so, why would the average user sue multisig.  i think it is prob something that we may all use further down the line once we see the benefit