The Spectre's fit very nicely. They have overalls that you put on with a buckle in the middle to attach it. The back part goes up under your shoulder pads and for me (I'm a shorter guy) the sides went right up to the bottom of my shoulder pads, which is perfect since that leaves nothing unprotected on your upper body.
The most important thing in protective equipment, especially rib guards is their level of protection. The outside shell of the Spectre's is made of a hard piece of plastic which has a little bit of flex to it, but for the most part is pretty stiff. Taking lots of hard cross checks and some slashes which hit where the rib guards sit (that's how we do in Buffalo!), I could barely even feel the hit. The Spectre's absorbed the impact very well, definitely doing their job.
You would think with the amount of protection that these rib guards give, they'd be pretty heavy, but they are actually very light. While playing I could barely feel that they were on, and they didn't limit my speed or range of motion at all.
The inside of the rib guards has Under Armour's Heat Gear layer on them. This wicks some moisture from your sweat and keeps you cool and dry (yes, it actually works like that). The thick plastic of the outside of rib guards isn't too breathable so you will be sweating while wearing them. The Heat Gear layer helps a lot with this issue, absorbing a lot of the sweat and helping you stay cool with them on.
The Under Armour Spectre Box Lacrosse Rib Guards are available for around $40 in most online retailers and are worth every penny. I recommend these to all box players looking to get better protection that is needed without sacrificing comfort. I also recommend these to some field attackmen who take a lot of checks in the unprotected upper body area under the shoulder pads and are looking to amp up their protection, but for most field players, the use of such heavy duty rib guards are unnecessary.