Why Use the Gnosis Multi-Signature Wallet
It is currently holding an extremely large sum of Ether and tokens.
Its code is simple.
It possesses an easy-to-use user-interface.
The wallet has been audited and bug bountied by the community on multiple occasions.
Many ICOs prefer using this wallet over other alternatives due to its simplicity and the fact that it hasn't been hacked...yet.
It can be used with a Daily Limit - if most of your keys are lost, you can still drain your funds out slowly every day with a single account.
It is open-source, and maintained on Github.
It has lots of coverage and oversight by the community.
Finally, Gnosis Team and ConsenSys does persistent maintenance work as well.
That being said, there are still potential issues with using the Gnosis Multi-Signature Wallet.
There may be vulnerabilities in multi-signature wallets that hasn't yet been discovered by the community. The Parity wallet is one such example - an open-source multi-signature wallet that was trusted by the community right up until its "smart contract" was hacked in July 2017 (causing people to lose over 150,000 ETH). The programmers hastily pushed up a new "smart contract", but that contract had a new vulnerability that led to another hack in November 2017 (causing people to lose ~513,000 ETH).
To use the Gnosis Multi-Signature Wallet, you need to visit https://wallet.gnosis.pm/. That website could be compromised by hackers.