I bought a stealth 1000w last year. I would love to build my own ebike but just haven't got the time to. I previously used a Revolver from Cyclotricity which I was really happy with - it's now used by my wife!.
You will need to sign a disclaimer to get a derestrict code from Cyclotricity to allow the 1000w mode to be used. It is not a simple single button press to switch between 'legal' and 'non-legal' modes and involves turning the system off and on again and going through a rigmarole of button pressing. You certainly wouldn't be able to do it quickly if you were involved in an accident or stopped by the police!
I found that not only was the original gearing (44/14) way too low for 1000w ebike speeds but that the chain was also so out of alignment in the highest gear that it was catching on the next highest sprocket and jumping all the time. After trying a few things Cyclotricity offered to pay for a freewheel remover so I could remove the spacer between the freewheel and the motorized hub to slightly better align the chain. This caused the rear derailleur to contact the hub when trying to select the uppermost sprocket. I have since purchased a DNP freewheel and just avoid using the lowest sprocket. I think the bike is designed to have a standard triple chainring type chainset and these problems have been caused by cost cutting - they have used a single chainring option meaning the chain will never be properly aligned in all gears - you would never use the smallest chainring and the smallest sprocket on a 21 speed bike for example. I plan to get a triple chainring at some point which should not only allow me to align the chain properly for higher speed ratios but also give me the option of much lower gearing should the battery ever run out. Trust me the direct drive motors are far heavier than the freewheel types and you wouldn't want to ride the bike without assistance - it actually has a slight resistance to rotation when unpowered as well.
On the plus side it is very fast if you want it to be - 30+ mph and gets there quick too
I have a 17 mile commute to work and only really use assistance level 2 (of 5) (which on the flat gives about 23-24 mph) plus a bit of throttle for the hills. I'm lucky that my commute is all along private land and I have the owner's permission - although if I were to go much faster I'm not sure if the battery would last the whole way - at 20+ mph a couple of extra mph can have a serious effect on range. I would imagine that the 500w Revolver version would be more than adequate for me but having the extra power available is nice.
To sum up it's a quality ebike kit married to a fairly budget bike I would expect for about £150 from Halfords. Taking everything into consideration (assembly, liability insurance and other overheads etc.) for the cost of it RRP I would say it's fair value for money. I got mine on the ride to work scheme so consider it great value for money - I priced up the bits they use (all available at BMS battery as far as I'm aware) but couldn't build it myself for the price it cost me so it was a bit of a no-brainer.
Also it's right what other people have said that they're a great company when it comes to customer support. If you're after a fast budget ebike with a good warranty without having to build it yourself then it's money well spent. If you want a good base bike you'd do well with a kit from them too and sourcing your own bike. Check out ebay because I think they're £50 cheaper or so on there.