Sunshine Kids Juvenile Products makes 1 car seat and 1 booster seat. However, their 1 carseat now has 10, yes 10 versions.
The Radian 65 and 80 started out rear facing up to 33 pounds and forward facing up to 65 and 80 pounds (hence the names). In late 2007 or early 2008, they changed the rear facing weight limit from 33 to 35 pounds. In Sept 2008 they added the Radian XT, it is a Radian 80 with added head wings. In early 2009, SKJP retroactively (back to seats made in/ after Sept 2008) changed the rear facing weight limit to 40 pounds for all 3 Radians (65, 80, XT). There were cosmetic and structural changes made to the Radians in Sept 2008, those are the seats that passed tests rear facing to 40 pounds.
Now in Sept 2009, SKJP has released a new generation of the Radians. The Radian 65SL, 80SL, and XTSL. The SL stands for Super Latch. The new Radians can use LATCH up to the full weight limit of 65 or 80 pounds (*if the vehicles allows it). Most vehicles have a LATCH limit of 40 or 48 pounds. All vehicles manufactured after Sept 2005 were required to pass a stricter test for the LATCH. ONLY vehicles manufactured after Sept 2005 can use LATCH with the Radian SLs up to the full weight limit. Confused yet? So are many of us :)
The Radian 65SL can rear face up to 40 pounds. The Radian 80SL and XTSL can rear face up to 45 pounds.
Now that you have all the info, I will give you my review :)
The Radian has 5 sets of harness slots; 8 inches, 9.5 inches, 11 inches, 14 inches, and 16.5 inches. The lower 3 can only be used rear facing, and the top 2 can be used rear facing or forward facing. There are 3 crotch buckle positions. The Harness adjuster is like a Britax in that you tug, stop, tug, stop a few times while tightening.
The Radian XT has head wings that slide freely. You simply push them up or down so they are right above the child's shoulders. However, when a child is using the bottom 2 harness slots, the head wings must be all the way at the top of the seat.
The Radian has a rear facing base that must be used when the seat is rear facing. Unlike most other seats, you can not use a rolled towel or pool noodle to help with the recline. Unfortunately, sometimes the base makes to seat too reclined (like in this picture), even for a newborn. There are a few tricks you can try to make it more upright, but it doesn't always work. Like Britax seats, it can be tethered even when rear facing.
The Radian installs very upright forward facing. It does have a recline feature, but it doesn't recline it very much. Because it installs so upright, children can often fit in the harness longer than they can in seats with taller harness slots. It can also be difficult to install forward facing in some vehicles.
The Radian has a nice tall shell. It should fit most children rear facing for a long time (usually at least 3 or 4 years). With the new rear facing weight limits, it can definitely keep kids safer longer :) The covers are nice and soft. There are several mesh covers, I don't like the mesh but I've heard it can help keep kids cooler in the car. The newer seats (post Sept 2009) even have comfort foam under the child's butt.
I think the Radian is a nice seat, but not the answer for everyone. I'd say try it in your vehicle before you buy one, or buy from an online retailer with a good return policy. I'm using mine for Sofi now, and I'm liking it so far. I kind of wish I had the head wings of an XT though, so I may end up with one somewhere in the future.
Some pics and info courtesy of members at car-seat.org