I hate reversing onto these. Even though they are portable and can be adjusted, Its when you get the odd bloke who wont move the ramp if you are an inch or so off!
Also remember someone getting stuck in the back of a container they was unloading after taking a good run up, Up the ramp, Slamming on after realising he was going too fast and shunted my lorry forward off the ramp. Needless to say i ended up nearly falling off the bunk in the lorry because of this!