There are several different schemes out there with some better than others.
I bought mine through my old employer who used Halfords as the supplier which seems to have worked slightly differently to the cyclescheme option.
1) Between me choosing the bike and buying (4 months) the bike was put in the sale. There was no problem with me buying it at the reduced price but I did find myself having to spend several hundred pounds on accessories in bikehut which wasn't a simple process.
2) At the end of the scheme my employer gave me the bike which meant I was liable to pay tax on the value of the bike. Sadly it was a one year scheme so I was paying tax on £250.
The maths worked out as
Headline cost £1000 (Halfords also added 10% extra in accessories)
Cost after tax and NI deductions (what I paid) £680
Tax pad at end of scheme £50
So I paid £730 for £1100 worth of bike and accessories. which seems like good value.
A couple of other notes on the cyclescheme one:
1) I think the full price limitation is only enforced at shop level so it may be possible to persuade a shop to sell you a 40% discounted bike for a 25% discount and still use the voucher for double savings.
2) According to the cyclescheme website you can use a voucher to buy accessories or safety equipment so if you're not in the market for a £1000 bike you could buy a £1000 frame or wheelset instead. Or alternatively buy whatever part you expect to wear out within the next year and then post them back with no further costs at the end of the year.
My final comment wont be relevant to most people but the £1000 limit is a credit licence restriction so if you're lucky enough to work for an employer with a credit licence there is theoretically no limit to how much you can spend.