I use Vani-T velocity and think it is lovely. It has 4 different bronzing ingredients so doesn't just use DHA to tan you. It is a 12% solution and will give you a 12% colour in 4hrs. Most tans that you wash off after 4hrs will not give you the full colour, you would most likely require a much higher dha, which 'could' result in you going orange.
Also not everyone will change colour if they wash their normal tan off early.
Velocity is handy for people who may want to get a last minute tan. They can wash it off 4hrs later and start to get ready, put makeup on etc. The colour still takes about the 8hrs to full develop, but they can wash and do hair etc much earlier and they just darken once they are out. Most will have a good colour by 6hrs, so if they have it at lunch time can be washed, ready and tanned by dinner time.
I find it is also handy for mums who need to get on with things like afterschool activities with kids. school pickup in the rain etc and can't really avoid getting wet.
Hope that helps, I find Velocity to be very popular
Jen x