They're impartial insofar as all the companies on there have to pay to be listed and it's possible to pay extra for premium visibility.
That's just not true.
http://www.moneysavingexpert.com/site/moneysavingexpert-financeI've been looking in to a re-mortgage recently and wouldn't use a broker. They're no different to PPI reclaim companies and the like. They're just doing something that you can do yourself with a little bit of effort.
If it's something complicated like a buy to let on a second property or a high LTV mortgage or you've had bad debts then yes a broker may be of use because he'll know who you're most likely to get accepted with so will save loads of time. Also if you don't really know what you want then a broker can offer advice.
If you've got decent equity in your house, a reasonable income and you know you want a 2 year fix rate deal then just do it yourself because it'll be easy.
If you're in the easy group then I'd take a look at Tesco or HSBC. They both do really good rates at the moment. To be honest though unless you know what sort of mortgage you want (fixed, variable, tracker) and how long you want it for / fixed for it's really difficult to know where to start.