Actually, just to make a list of what the rare items are capable of:
Sniper - Self-explanatory, though the thing that makes it fairly special is that it has silenced rounds and it can zoom in twice rather than the typical once. Does quite a hefty amount of damage even nerfed if it gets a headshot. This can be holstered.
Flash Grenade - Self-explanatory once again, the flash grenade creates a flash that impairs the vision and hearing of anyone within close proximity. Holsterable.
Smoke Grenade - This creates a smoke screen that lasts for about half a minute. Quite effective at distraction, though the flash grenade would arguably be a bigger distraction. There's currently a glitch where throwing one causes two to be thrown while only counting you having thrown one. So, quite a big of fun there. People claim the FPS drops when in use, sooo... yeah. Holsterable.
HE Grenade - Arguably the rarest grenade at the minute. Not hugely effective, only does about 100 damage at most. It's a quirky grenade, in the sense that it bounces everywhere when thrown. Holsterable.
Cooler - The only completely useless item of this list. Currently rendered obsolete by the fact that the heroic cooling upgrade has a similar effect. The Cooler had an original function that was to protect the printer from exploding up to 4 times. There are very few of these remaining. Fun Fact: This isn't even a proper item, anymore. Hence why when people look at the model for it, it's depicted as an error.
Crack - A 300 million - 600 million dollar paper weight, for the most part. It has an effect, but the effect is the same as that of Amphetamine. It speeds you up for a short time. If you use it, you might aswell be wasting all that money.
I say this as someone who has all of these items, already. I think they're highly remarkable items, just not all that useful given their "high risk, low reward" status.