After some research my wife and I bought a Jane Slalom Pro Matrix travel system for our baby daughter (born November 2005).
This review concentrates on the Matrix seat, I'll do another one for the pushchair/chassis so that I can be more specific in each.

- We liked the chassis
- We wanted something that our little girl could sleep in on long journeys (London - Scotland), having read about the recommendations about small babies in car seats
- We liked the idea that it would later serve as a group 1 car seat
- We thought it made for a nice, traditional type pram on the chassis
- It does the above without having to buy a somewhat more expensive pushchair+car seat+carrycot system

What you get;

The seat can be adjusted to 4 positions from flat (group 0 car seat) to 138 degrees (group 1 car seat). This is simple to do using a single handle on the back to unlock it and can be done "on the fly" whilst the seat is fixed in the chassis and the baby is inside it. The handle is likewise adjustable to 4 positions with an unlocking lever on the top. One of these positions is for impact protection when used as a group 1 car seat.

For its purpose as a pram/long journey car seat the Matrix has been brilliant.
In pram mode it looks really smart (very important when your child is wowing old ladies...) and you have the added benefit of being able to sit your baby up or lie them down as required. This is coming to the fore as she gets older and wants to look at things on the move or while we're stopped to eat. The handle is simple to adjust to get it (and the hood) out of the way when sitting up/wowing old ladies or to put toys hanging from it in the right position. The "ProFix" system makes it a doddle to remove from the frame. A single button on the top of the handle unlocks it, replacing it is a simple "drop in". There are no additional clips or adaptors required. Whilst the Matrix is on the Slalom chassis the pushchair can still be folded almost completely. The locking catch won't quite engage, but it still makes it more compact for "parking" in a corner. (I have made use of this to get it through tight spaces, with the baby strapped in & grinning!)
As a car seat it can be fiddly. This is the main reason for deducting a star, I think that some of the nuisance points could be designed out fairly easily.
Lying flat it will probably take up all of the rear seat unless you have a VERY wide car. This is because of how it fixes to two seatbelts rather than its actual length (in case you wondered). It fixes to the belts using clips which can be left on them. Getting your head around which way to point them on the belt can be confusing (it got me) - look at the last picture in the manual. They're quite simple to fit to the seatbelts and allow them to be used normally. In theory they're quick & simple to fit to the seat. In practice I've found the one at the foot of the seat to be a *&^%$£@ (tip - line the straight edge of the clip up with the lip of the seat to give yourself a fighting chance and remember to loosen off at least one of the belts before fitting) - designers fault entirely. I've spoken to one person who just leaves the clips on the seat and fits them to the seatbelt each time. Bear in mind it's quite large & heavy (especially with a baby fitted) if you are of smaller stature. On the plus side it looks like it should fit to just about any rear seat with belts at either side (I wait for someone to find a really extreme bucket arrangement that proves me wrong...).
Our baby sleeps in the seat this way as happily as she does when it's a pram.
We haven't used it as a group 1 seat yet but it's easy to convert. Also worth noting is that you don't need the clips for this - you just fit a seatbelt around it. This could be handy for getting lifts back from long walks. The harness adjusts in the usual way (push a lever by the catch and pull the straps). I found this difficult with it laid flat, but you're not supposed to use the harness for that position.

In summary;