In my begginer days, I used a bike like that. What a horrible experience trying to ride single track on it I had.
Firstly it weighed a tonne. Climbing on road or off was a miserable experience. The rear suspension was soft enough that it compromised traction on all terrain but smooth tarmac. It sapped atleast a third (perhaps half) of all the energy I was putting in peddling. Wasted on bobbing around.
This is rather disconcerting at first but did actually challange my balance skills on anything technical where the bumps were big. Still, not confidence inspiring and not what a proper bike should do.
Also, the rear suspension pivot points where so poorly constructed, that despite tighting, there was about 3mm's of movement at the pivot point next to the bottom bracket. 3mm's doesn't sound a lot, but translated back down the chain stays to the wheel drop outs, it results in the wheel moving up to 2 cm's off of dead straight, making for some perculiar cornering behaviour.
The front suspension also bobbed excessively, robbing me of further effort. The components were rubbish, gear changing loud, not at all smooth and just horrible. This thing had awful V brakes, and plastic brake levers. Absolute rubbish performance that forced me to limit my speed and not enjoy myself as much as I might otherwise.
There was a little give in the bottom bracket / crankset. With my weight on the drive chain side pedal, it moved a few mm's. More wasted energy.
Just about every moving part creaked and groaned, sounding as if it was operating at the edge of specification. I am not a big guy at all. I don't know my weight but its probably around 10 stone as I am 5' 10", and moderately built and not overweight.
The accumulated experience left me nervous and fearing for my life should the brakes fail!
Halfords invariably sell toy shop bikes. Go to a LBS and get a entry level model from a big name brand such as Trek, GT, Giant, Specialised, Kona etc. You will get a very solid bike.
