I'd say to anyone thinking of PIP to go for it. You won't know if you don't ask.
I know everyone advises people to get help completing the form but from my own personal experience I'd only follow that advice if you really feel it necessary.
The people who provide help filling out these forms use a very formulaic method such that pretty much every form they are involved in looks and reads much the same. They stand out a mile to the staff who receive them (again speaking from experience of working in a housing benefit office) and they tend not to reflect the reality of that person's issues and life. A lot of them are rejected without even being read fully (much like CVs or book manuscripts that follow a formula).
I'd say it is far better for the person affected to complete the form themselves if they are capable, referring to the notes supplied with it and information available online (there is a wealth of advice and info available for free online) and maybe get a friend to check it over afterwards.
Obviously if the person isn't capable of doing this then they need to get help.
Completing it themselves makes the information more authentic, honest and relevant. It also means that when that person is called for an assessment or visited at home they should have an idea what was actually put on the form, as the assessor will be looking to catch them out on everything on there.
From my own personal experience I have done my own forms, without any assistance, for the past 11 years. I've never been refused DLA and now PIP, never had to appeal and always got the correct rates. I've also never used buzzwords or tried to pad answers out for the sake of using up space - some of my answers to the questions on the form have been just one word.