TLDR version: zero star product, unfit for rough cutting, not worth buying at any price

First impressions aren't too bad: the body feels very solid, the plastic is hard but the parts fit together quite well. There is no riving knife, but the spring loaded blade guard works and appears to be safe. Starting it up it's quite loud, but not unbearable. However, once you start cutting the limitations quickly become apparent.
The flex cable: predictable low quality and resists flexing freely, but it's to be expected at this price and might be forgiveable if it wasn't so damn short. Pulled taught it's not even 2m long! A real pain in arse as soon as you need to cut a 2400 × 1200 mm or 8' × 4' panel. Extension flex and some apparatus to prevent snagging is essential.
Cutting some 18 mm OSB panels: my first use of the saw I only needed a rough cut, so did it freehand without using any sort of guide edge. For the first meter or so the cut was relatively clean and the only issue I noted was following the pencil line free hand as the saw felt a little unwieldy. However, further along I found the saw more difficult to control, vibrating more and somewhat bucking and twisting. The final cut was just about straight enough but not very plumb, with maybe around 10° of unintended bevel in the worst areas. Initially I just though the guide base was a bit too flimsy and soft in the afternoon sun, coupled with inaccurate markings for the depth and bevel. I thought I could compensate for these issues and gave it another go on a cooler day and indoors...
Cutting some 5.5 mm hardwood ply: I set my sheet of material down so that it was securely clamped underneath some heavy sheets of OSB to stop it moving and vibrating, the straight edge of the OSB doubling as a guide edge. Not needing it to be perfect I checked the bevel by eyeballing the blade against the base, looking back at the markings on the saw it was slightly off zero. The blade and guard looked to be slightly off parallel with the base, but I figured it would be adequate to cut, albeit with a rough finish. When attempting the cut it was immediately impossible to keep the saw against the guide edge that I'd set up, the saw just wanted to constantly twist clockwise away from the guide edge. The wood started smoking a lot. Overall the least straight, most ragged cut I've ever made with any type of saw in any material, truly embarrassing.
After cutting the plywood the base of the Energer saw was hopelessly twisted as can be seen in the photos. The strut with the cut depth markings is bowed out of shape and although the base twists easily with the soft, thin metal and can't actually get it back into anything like a true position. Inevitably I will be asking Screwfix to accept the saw back for a full refund as I just don't think it's fit to do any sort of cutting.
N.B. I also have the Energer Jigsaw, it's crap but a bit better than the circular saw and OK for DIYers to make rough cuts only, but tradesmen would likely find it unbearable due to the amount of vibration. I will post some more detail comments in a separate thread at some point)