In reply to Bog ninja:
I've got the D'Lite with suspension, used for the nursery / school run through the local woods as well as some longer family adventures.
Can't directly compare to those without suspension, but in summer in particular, over the really churned up, hard mud - I imagine the suspension lessens a lot of the harsh, quick vibrations, as well as soaking up tree-roots and holes. Never had any complaints at least.
It's also adjustable depending on the number & weight of kids you're towing.
They're not cheap (at all), BUT do hold their value incredibly well. I think I bought ours for £600, and after five years of use can probably sell it for £300-400 looking at eBay.
The only thing that does annoy me specifically about Burley is the cost of accessories; a second attachment hitch for my wife's bike, which is about 30p worth of metal is something daft like £15, when at the price of the trailer should be included.
Likewise if you want a jogger wheel or pushchair wheels, these are silly prices too.
Thru-Hubs aren't that cheap if you buy the Burley version (I had to replace mine as the end just sheared off after 3 years - went with third party which was £5 more, but looks much better build-quality).
Last thing to mention though is that is was SUCH a great thing to take to a festival (remember those?!), push-chair / seat for tired legs in the day with loads of storage space, and a cosy bed in the evening whilst we partied into the night.