Have used Dickmanns quite a few times over the last two years and never had any problems. As others have already said sometimes not the newest of cars and some don't come with parcel shelves. Haven't always found them to be cheapest, particularly when booking direct rather than through an agent (although you do get cost of full tank of fuel quoted when going direct), but staff are always friendly and don't give you the 'hard sell' on extra insurance etc. like some do.
Enjoy your hols, we are out for 2 weeks from 25th so may bump into you! (not literally I hope)
Regards,
David