This does what it says on the tin: it's a pen, that writes like a pencil, but doesn't have a lead that can break or needs sharpening.
I got mine off the shelf in Hobbycraft (they have a big Sharpie display) and immediately after unpacking, I noticed a tiny bit of sealant on the nib - something to watch out for when opening a fresh pack.
It feels very much like a smooth ballpoint pen when writing, actually quite pleasant, but the marks it leaves behind are graphite grey which is a very odd experience initially. Sharpie claim a #2 lead - HB - but I'd say it was more a 'B' (#1) in weight.
There is a little unevenness in the line weight of the markings, but this is consistent with using a pencil. Like a ballpoint, the line thickness is defined and doesn't alter as you write. I find if you don't use the pen for a while it does dry up a bit, but like any ballpoint, you can get it going again with a quick scribble on a scrap of paper.
The top of the pen has a prominent eraser - like those you get on mechanical pencils, except this is exposed and not under a little cap. There's a good reason for this - it feels natural to rub out mistakes with this pen, and it works, too. Because the 'lead' is quite dark, erasing isn't quite as successful as it can be with a regular pencil but effective nonetheless for the odd correction. After a couple of days, your writings become permanent.
With the twin pack, Sharpie include six spare erasers - I dare say to see out the life of the ink. In reality, any eraser will do.
This pen has a nice silicon grip on the barrel - the only odd design choice is that when the nib is clicked down, the top clip clicker assembly is loose... to be fair the buttons on most biros are like this but still it's a bit odd.
The Sharpie Liquid Pencil ultimately is a product no-one was asking for - pencils, mechanical or otherwise are cheap, easily available and work. Apart from addressing the issues of broken leads and sharpening... these are things you just get on with while using a real pencil.