Is there a reliable way to see what other users see when logged in?
I can't vouch for either of those modules, but I've been using the 'Switch' feature in the Devel module to do just that for years. I've used it on probably hundreds of sites and I've never found a problem with it.

When logged in with appropriate permissions you can visit /devel/switch/[username] and it will log you in as that user. There's also a list of users in the admin toolbar that you can choose from directly, but I'm not sure if that's only available if you have the Admin Menu module installed.

It doesn't add a link to log back in to the admin user like the Masquerade module does (it's more 'bare-bones' than that), but if you've got your admin password saved in the browser it'll only take a couple of clicks to log back in anyway.