I have recently embarked on the journey and my situation will be different to others as we went for and were approved a lump sum IVA, however it wasn't the initial plan and plan b would have been the standard IVA.
I have no experience of the annual checks etc as it is all very new to me, but the level of service received so far, would tally with Gary post above.
Michael Peoples from the firm contributes on these forums regularly and I would recommend dropping him a line to get a view.
There is no harm in sending your information to various firms and asking for their opinions and their calculations of I&E etc how easy it is too talk to the same person, how often there are bad reviews/moans on here, how they handle PPI etc etc just DONT send the "authority to act" to one firm until you're sure, sit on it for a while once you have had the same conversations with various companies, this is what I did and and in the end I was very glad I didn't go with the company I was initialled referred to by CAB (I then found this site in the nick of time that helped be learn lots more and re-consider based on consumer experience).
My recent posts about IVA approved is here in the approved IVA section.
http://www.iva.co.uk/forum/topic.asp?TOPIC_ID=69752 or on my blog link below.
Good luck.