http://www.madrockclimbing.com/products/product.asp?_item=100161


tl;dr: It's a very good copy of the Grigri. My Grigri 2 now dwells in the gear closet.
The Lifeguard is a little bit lighter than a Grigri2, 156g vs 172g according to my kitchen scale. It's substantially smaller however, in a way pictures don't really convey. When I took it out of the box I couldn't help but laugh at how tiny it is. It's entirely metal though, no plastic parts at all, so it kinda "feels" heavier than the Grigri 2, because it's denser.
In terms of MSRP it's slightly cheaper than a Grigri 2 at $90 vs $100. Practically speaking though Petzl stuff goes on sale so often compared to Mad Rock it's more like $90 vs $80. I'm allergic to paying full price so I used a Campsaver coupon code to get it for $72.
But never mind all that, how does it work?
Mechanically it is absolutely a Grigri copy. The sideplates open the same way, the rope moves through in the same orientation, the cam pinches the rope in the same place, the lowering handles operate the same way, and so on, and so forth. The only practical difference I can detect is that, due to some combination of spring tension and cam geometry, it's markedly less prone to locking when paying out slack quickly. This depends on rope diameter, of course. I usually use it with my Trango 9.9, with which it works well. It also works well with my dry-treated Sterling 10.1, but a well-worn non-treated Sterling 10.1 gym rope makes it a bit more lock-happy. It belayed beautifully with an Edelweiss 9.5, but lowering friction was a bit scary. If it does lock when you don't want it to, it will typically unlock the instant tension comes off the rope. So if your partner, when short-roped, simply holds tension on the rope waiting for you to push the Grigri cam open, you may have a bad time, as the cam on the Lifeguard is smaller and a bit harder to find blindly with your thumb. If, on the other hand, your partner responds to being short-roped by giving a bit of slack back to you, the Lifeguard will unlock without direct intervention and you can feed slack normally.
The marketing fluff says you use it exactly like a tube device, with no need to learn any special technique. I'm going to rate that as "nonsense" on the truth-o-meter because even though it's less prone to locking up than a Grigri 2, it does function exactly like that device, and will at some point lock up when you don't want it to, so you need to learn the technique anyway. If you're already proficient with a Grigri you will be just fine. If you've never used a device like this before you'll need a bit of practice.
Lowering, compared to a Grigri 2, is kinda better but kinda worse. The lever is short, narrow, and doesn't open to as extreme an angle as the handle on the Grigri 2, so some users will find it uncomfortable. I don't mind it though, and find overall it gives subjectively better control than the Grigri 2. It is compatible with the Petzl Freino, if you happen to have one of those.
For top-rope belay it's perfectly fine. The Grigri 1 is still better for TR belay on stiff, fat gym ropes.
One quick note on carabiner compatibility: I recommend NOT using the Lifeguard with the DMM Rhino. The "horn" on the Rhino, which effectively prevents the Grigri 2 from rotating onto the spine of the biner, fits into the Lifeguard's slightly larger biner connection hole, and the Lifeguard can get stuck on it. This isn't just theoretical, it actually happened to me during normal use. Another time the Lifeguard rotated onto the nose side of the Rhino. It wasn't stuck exactly, but it stayed there when loaded.
So basically it's a slightly improved Grigri 2. I like it.