I received my Fleshjack, an Ice Jack: Ass - original sensation, a couple of weeks ago. I'm going to need some goes and time with it before I can really give a verdict on it, partly due to my infrequent sex toy usage where I'm at (living with relatives) and partly due to my current conditioning (more on this when it comes to the sensation). I received it the evening before going on Easter vacation with my relatives, and gave it a go that same night.
First off, the Fleshjack / Fleshlight sleeve material is exceedingly soft, pliable, and low on friction; it's unlike any material I can recall touching. So, at least for me, it took a little time to get conceptually used to this *thing* I was to stick my dick in.
Second, you use a hard plastic case to hold the material in place so that the "ass" in question doesn't fall to one side. If you have small hands like I do, you may want to use both hands on it as it's fairly volumous. Getting the sleeve into the case was tricky, though it might get easier with practice.
Third, you should rinse the sleeve before your first usage and each time after using it (unless you have an interest in keeping whatever cum and lube you might have put into it). Use lukewarm water; hot water will deteriorate the sleeve material, increasing its friction and potential stickiness (a mistake I started making when I first washed it).
Fourth, lube up your erect/semi-erect dick and the sleeve itself. The instructions say that oil-based lube will damage the material (yes, it's fragile), so you should use water-based lubes with your Fleshjack. Once lubed, go to town with it.
Now, if, like me, you're used to jerking off with the infamous Grip of Doom / Deathgrip, you will probably find the original sensation* Fleshjack way softer than your masturbation experience. In my case, I was uncertain if I could even cum with such a comfortable "grip" on my dick. If you think the sleeve material feels odd in your hands, just wait until you're inside it, jacking off with it.
In the end, I enjoyed the odd sensation, came with it, and had an easy time with the clean-up. The only problem I had with cleaning it was that the sleeve material is a dust magnet, causing dust and stray hairs to stick to it even when dry. If I took the time to practice with it regularly, I think I'd prefer having it as my standard jerking-off tool and using my Grip of Doom when I just need a quickie.
I should note that, at the time, I was also penetrating myself with a six-inch replicock, which made the experience more clumsy than it had to be and may be part of why I'm not ecstatic about my first experience with my Fleshjack. Because of this, I don't consider myself as having really experienced the Fleshjack to its potential and therefore consider this as a part one in a longer reflection on it.
(* I wish I had ordered a tighter sleeve, and recommend considering it if you also use the Grip of Doom when jacking off and are planning your first Fleshjack/Fleshlight purchase.)