the qt client, while being the baseline of measure, is a dinosaur. I use Electrum because it does not require the maintenance of a local blockchain, start up is immediate after long darkouts. It can be fully run off-line with no network connectivity. It supports aliases like email addresses instead if complicated bitcoin addresses. It uses deterministic key generation so I can literally backup my wallet in my head with a passphrase, erase my wallet, and recreate it from my mind. Oh yea, and it allows me to make transactions without any fees. Other worthwhile clients: Armory, Multibit. They are all viable and under rapid development.

Edit: electrum can be fully run offline as in you can monitor your balance without a network connection AND you can write transactions that can then be injected into the blockchain by a networked connection. But the electrum client at no time needs to be connected to the network.

Additionally, a networked computer can contain a deseeded wallet (no keys) for securely monitoring your off-line balances.