I usually like to install skins on thin phones like the Razr and my last phone the Xperia Arc, as bulking it up with a case seems to defeat the whole point of the phone's design. Anyways, have a Skinomi on my Arc, and decided to try a Xtremeguardz skin on my Razr...mostly because the Xtremeguardz skin was much cheaper at $5 vs. $15.
Whereas the Skinomi had entirely seperate pieces for the sides, top, and back, the Xtremeguardz back piece wraps around the sides as well...there's seperate pieces for the camera area and very top. Personally I'm not sure I like this full wrap around design, as what I've found in the past is I tend to get annoyed with having these skins stuck to the sides of the phone. The sharp edges tend to catch on my holster case I clip to my belt when sliding the phone in and out, and before long it starts looking chipped at the corner edges. It also tends to pick up dust and debris on the edges and never stays clean.
On the Skinomi for my Arc, I had the option of just simply installing the back battery cover skin, and I opted to leave the rest bare...this worked the best for me as I wasn't constantly feeling the sharp edges nor the stickiness of the skin, and my main concern was having protection for the phone's back when I set it down on surfaces. My leather holster protected the rest of the phone from daily abuse. My Xtremeguardz on my Razr still seems to be curing its bubbles after day 3, and I find also the edges keep getting unstuck on the Xtremeguardz. I haven't started using the phone yet daily, so I'll have to see if I want to stick with using it.
As far as the included screen protectors go that come with these skins, I never bother using them. I find they're all just too sticky on fingers to be used on a swiping surface, so I always stick to regular hard screen protectors to get the smooth gliding sensation.