While I like the Go Dev Team's launcher and launcher-specific app-replacing widgets, the team's Go Locker lock screen replacement app isn't for me. It's a great lock screen replacement, but is mostly based around ready made themes, and doesn't offer much customization outside that. WidgetLocker on the other hand is an app that is more complicated to use, but also awards users for taking the time to sit down and learn it.
Lock screen replacements are apps that replace the Android device's default lock screen. Not the pattern/pin unlock screen, but the lock screen layer that you can enable before that stage. WidgetLocker's "thing" is that it allows you to use the lock screen much the same way that you use a launcher, including adding widgets to it. That opens up for so many options in terms of customization, and you can glance at widgets without actually unlocking the phone.
If you start WidgetLocker through the app tray, it starts in edit mode, which is very much like the edit mode on launchers. You can click and hold to add widgets, resize them, and access all the settings that will be hidden when your device is actually locked. It really is so much like using a launcher, you can even customize the icon grid which determines the size, position, and number of widgets you can use.
You can access all your Android-level widgets, meaning widgets that aren't restricted to specific launchers. That means no HTC Sense widgets for HTC users, no GO widgets for GO Launcher users, and so on – but most widgets will still be available. You also have a few WidgetLocker-specific widgets, basically a customizable clock and the actual unlock sliders. The click is nice because it has fields for owner information, allowing you to have your name and contact information visible on the lock screen. I put my email address there, since anyone who finds my phone will have trouble reaching me with my phone number…
You also have a myriad of settings, both some that are a lot like launcher settings and some that are more lock screen specific. Examples of the former include wallpaper and transition animation, examples of the latter include locking buttons and widget taps to make them more lockscreen friendly.
As for themes, you do of course have that as well, it's just that it's an alternative rather than the only way of proceeding. The interesting thing here is that themes can be had from two places, either Google Play – which only has a few – or a forum thread on XDA Developers. The forum thread is linked to in the app as an "official" theme source, and I'm a sucker for community-based apps. There is a ridiculous amount of themes in that thread, so there should be something for everyone. There's also a Google Play app that helps you browse themes without visiting XDA. WidgetLocker also integrates with ADW launcher theme packs, allowing you to use icons from those – as well as your own images and icons.
WidgetLocker can also tie in with some third party apps, like Tasker and Unlock With WiFi. The latter is an app I use myself, and it allows you to automatically disable the unlock pin/pattern when in range of a network you've set as secure, in order to not have to use that extra level of security in places like your own home (requires one normal unlock each time you come in range, to avoid your WiFi being used to unlock your phone if stolen). The WidgetLocker option in Unlock With WiFi allows you to enable or disable WidgetLocker based on whether or not you're in what Unlock With WiFi calls locked and unlocked states. This opens up for a whole range of new uses for WidgetLocker, like having WidgetLocker be active at home when the code lock is not, or only use WidgetLocker when you're not at home or somewhere else "safe". That way you could e.g. use email widgets that you normally wouldn't want visible on your lockscreen and just disable WidgetLocker completely when you're out and about, or use the WidgetLocker clock's owner information fields to display owner information only when you're not at home, or simply avoid having to go through both WidgetLocker and your unlock screen by having one by on while the other is off. I just wish that it was possible to have multiple WidgetLocker profiles and use Unlock With WiFi to swtch between them, allowing you to have different lockscreens for home and not home, instead of being limited to one.
As for my personal WidgetLocker setup, I simply used the built-in stuff to create one that works for me. It uses an ICS style unlock slider with four options (depending on where you slide it): unlock, camera, Gmail, and LED. The first three will prompt for a pattern unlock if I'm not at home, while the last one simply activates the camera LED to be used as a flashlight and doesn't require unlocking the screen. I also use the included clock widget to display owner information, clock, battery status, and next alarm. That's just my setup though, you can basically make it do whatever you want. It's also worth noting that since this supports normal widgets on the lockscreen, it allows you to have media player controls on the lock screen without having a media player that has that feature built in.
All in all WidgetLocker is a great lock screen replacement, a one that is extremely powerful if used correctly. The amount of settings you can tweak can be offputting to some, but the default settings make enough sense for general purpose use that simply knowing how to add widgets to a launcher will give you the basic knowledge you need to deal with WidgetLocker as well. It's available for $3 in the Google Play store. As with a lot of cool Android apps there's a chance of both extra battery drain and random issues and incompatibility, so reading through the reviews for any mention of your specific device is a good idea.