So, the Cricut Mug Press is a sublimation heat press specifically designed for mugs and glasses that have a sublimation coating and fit within the dimensions of the heating area. In general, that means a diameter of 3.2" to 3.4" and a height of 4.72". However, you can work with taller surfaces, such as water bottles and beer mugs, but you'll be limited to the area the heating element works in. In some cases, that means inverting the mug you want to sublimate.
Now, Cricut does make their own sublimation mugs, in two sizes (12 and 15 ounce), and they're pretty reasonably priced. I could not find a cheaper option, Amazon or otherwise, and these are great mugs that are microwave and dishwasher safe.
So, you can off brand and will have to for drinkware that isn't a coffee mug, but it definitely needs to be designed for sublimation work and that means a polyester coating for this purpose. As with clothing, the sublimating ink needs a polyester surface to bind to. In any event, these are readily available.
How does it work? Really easily. When you first get your device, connect it up to a computer and run the setup. The sole purpose of this is to basically register the device so you get notified of updates for it. After that, it's standalone just like the other Cricut heat presses. To operate, you turn it on and let it heat up, it beeps when ready. Lift the handle, insert your mug with the design attached, and press the handle down. The device will begin working and will beep when done, about 6 minutes or so.
Now, the mugs coming out of there will be extremely hot. If you're working with a basic coffee mug that has a ceramic handle, the handle will cool, but the body will be very hot. For other types of drinkware, handle with heat gloves. You will let the mug cool for about 15 - 30 minutes and then you can peel the infusible ink or sublimation backing off.
With respect to your source options, I have so far only used sublimation prints from my printer, I have yet to use infusible ink (which they are designed for) and these have printed perfectly. The big advantage of the sublimation printer is that my designs can be more complex than working with infusible ink cuttings from my Cricut Maker or Joy. Having said that, there are some really cool patterns for infusible ink and the Cricut Joy is perfect sized for this.
Why go with this rather than a traditional mug press? For me, it was ease of use and safety. The alternatives are designed to be bolted down and the entire device gets very hot. The Cricut option only gets hot inside the heating zone, the rest of the device stays cool to the touch. It's much more portable and is more easily stored away. A good general option I think and not really more expensive.
Aside from mugs and sublimation sources, you probably also want heat tape. For infusible ink, the backing is sticky and you might not need it, but for sublimation paper, it's a must. I have the Cricut brand stuff, works awesome, but there are lots of choices.
Verdict? I love it. If you want to make custom mugs and want them to be permenant, this is a great way to go!
My first couple of mugs (pardon the glare):