I bought one from Halfords a few years ago... it was a full suspension bike, looked amazing, and it was only £50!
I went on a ride with my bro and my bro-in-law (who both have relatively good bikes - £250 & £750), and they're both 15 years older than me, and i thought i was gonna die...!
It was a dead simple ride - 26mile former railway line so completely flat, and after about 17miles i was ruined. At one point they were 15mins ahead of me and they stopped and waited for me to catch up.
At that point we swapped bikes, and they both instantly started whinging - we did a free wheel test... pedal upto the same speed and then stop pedaling and see how far you coast - and my bike was managing 10yds compared with their 60-70yds!!!! Thats why i was in such a state!!
Needless to say i didnt take it out on any more long rides..!