They certainly don't come across as flimsy.
That said, I would rather use Google Authenticate (TOTP) for two factor. Getting my phone out is a regular thing. Getting the yubikey out and plugging it in seems more of a hassle. Passpack is the only thing I have to use the yubikey for -- would be happy if they provided TOTP. Spending $40 for two factor these days is kind of ridiculous.
Its not only more convinient, its also far, far safer then TOTP.
The will not support TOPT because that would require constant power.
You can get a U2F only stick for 18 bucks from them. Once in a while (for example when github interduced U2F the sell 2 for 5$).
https://www.yubico.com/support/knowledge-base/categories/art...
Then there's the concern about who has access to those keys when I'm not around.
I keep a backup key in a safe location, and my primary on my keychain which is typically in my pocket.