is it convenient/reliable/secure?
Python isn't exactly my favorite language (only because I've never taken the time to learn it), but I've been using Electrum for a while now and it's easily my favorite Bitcoin client. To answer your questions:
- convenient? Create a wallet, connect to a server and you're good to go. You do have to download the blockchain headers for the built-in SPV transaction verification to work, but you don't have to download the full blockchain and you can spend any coins you receive as soon as they're confirmed on the blockchain. The wallet is deterministic, so you only need to back it up once (or just back up the seed, which allows you to re-create the whole wallet). To me, all of these things are convenient. It looks like BIP-32 is coming soon (the code is already there on GitHub and enabling it on my local copy was a matter of changing/adding about 10 lines of code), which will probably require the minor one-time inconvenience of moving your coins from your old wallet to a new one, but that's a small price to pay for the added convenience of being able to use your seed to re-create the same wallet on any BIP-32 compatible Bitcoin client (MultiBit should support BIP-32 soon, and there's already a pull request for bitcoind/bitcoin-qt to add most of what's needed for BIP-32 wallets there).
- reliable? It's as reliable as the Electrum server you choose to connect to. There are quite a few servers available and if your preferred server is down you can optionally tell Electrum to randomly choose another server.
- secure? There is one small security tradeoff, any server you connect to can potentially know every address that belongs to you. Other than that, it's pretty secure. If you use it as an online wallet with a password to protect your seed, it's as safe as using bitcoin-qt online (in terms of keeping your money from being stolen, at least). It also supports seedless "watch-only" wallets and offline wallets, so you can keep the seed for your private keys completely offline. With offline wallets, you create the transaction with your online seedless wallet and then transfer it to your offline machine for signing, similar to how Armory offline wallets work.
edit: I recommend enabling "Expert mode" which gives you a few extra features that can come in handy. With expert mode enabled you can see your change addresses, you can freeze an address to prevent spending any coins that belong to it and you can prioritize an address to spend the coins that belong to it before spending any coins belonging to any of your other addreses. There are probably a few other expert mode features that I'm not thinking about right now because I either never noticed them or never used them, but the freeze/prioritize is useful since it gives you more control over how you spend your coins.
