There are quite a few threads in the bike forum but for what's worth….
I bought a weeride seat to use (sold it today) but as I bought it (off ebay) around Christmas time and the weather has been pretty poor it was never used so I can't comment on that too well. Some people love them, some find them a bit difficult to pedal with.
I sold mine as I've just picked up a second hand Chariot Cougar 2 (a double buggy you tow). Chariots are the mutts nuts when it comes to trailers but you pay a lot for them.
The key thing you will find people talking about is the ability of the child to hold their head up, and with the added weight of a helmet. There is an article on the web I came across one day that didn't recommend any form of biking / bumping your child around until they were a fair bit older than 1. Others say it's OK.
I suggest you search the forum for Chariot, Croozer (an alternative trailer) and google them as well. Check out the Chariot facebook page for a few comments and links to other sites.
invodo.com has videos of the various Chariot trailers that are handy. If you have deep pockets and want a new one, let me know and I can send you a few details of several places in Europe.
smurf