I have owned this seat for three years. My daughter who uses it is now 7 years and 53 lbs. We use it in two older vehicles, a 1997 Subaru Legacy Wagon and a 2002 VW Eurovan.
I love this seat and feel very secure when I give it to the grandparents on her visits. My in-laws also have one in their newer Lexus SUV. I don't have an opportunity to use the LATCH anchors because I don't have them, but my in-Laws do, and its been nice because they don't usually have kids in their car, so the seat doesn't slide around. It stays in place. The backseat of the Lexus is narrower than my two vehicles, and it fits this booster and a Britax Roundabout directly next to it.
My parents hate using car seats. They reminisce about the days when I was a child and we could just lay on the floor of the car or stand up surfing in the back of the pickup. So I have to monitor their proper use of the seats carefully. The seat belt guide on the Sunshine Kids Monterey has a snug slot opening through which the seat belt threads. I admit, when I am in a rush moving this seat to another vehicle, it can be frustrating to get it in quickly. One needs to place it in the opening sideways and then slide and jiggle it back and forth to insert it completely into the belt guide. But then I know it will not slide out, and once inserted, it slides easily through it and does not get hung up or stuck. It is bright red and cannot be ignored. The seatbelt will not lay properly if it is not threaded through the belt guide, so I know that my folks will insert it, and it will not slip out. I also like that the proper position of the lap belt is bright red, as well, so they can't miss it. It it quite easy for anyone who has never used a booster to see exactly where to thread the car seat belt.
It is very easy to move the headrest up and down, and the shoulder belt guide moves with the headrest, so it is always at the right position on your child. I have had carpooling guests use this seat and I can easiy reach behind pull up the headrest to their height, even with them sitting in the seat. Adjusting the width of the shoulder sides requires you to reach behind the seat, so it has to be empty to do that. It is very easy to turn the knob which expands and contracts the width, but I have found that, over a few days, if your child wiggles or plops into the seat against the sides, it will open itself a bit wider than might be necessary for your child. When my child was smaller, I frequently had to do a quick reach behind to tighten the shoulder width if it has slowly widened itself.
This booster can be taken apart, allowing you to use only the base. It is really easy to separate and put back together, but it doens't fall apart when you move it from seat to seat. That is not to say you can just hold onto the backrest and expect it to stay together, both the bottom and top need to be held when carrying it around, but the backrest doesn't just flop off the minute you pick it up. For a year I carpooled with a neighbor who had two Graco boosters. Every time we moved those from car to car they would just fall apart, and it was difficult to piece the top and bottom back together.
The cupholders and open and close easily and are large enough for her kid-sized water bottles and a small baggie of a snack. The arm rests are stationary, but she usually uses them. I have never though of them as being in the way. The fabric on my model is a mesh grey. It is comfortable on hot days, not sticky or wet. It says cool and the mesh airs out when you get out of the seat. But I could do without the mesh, which allows tiny crumbs through and takes a bit longer to vacuum out. The padding is ample and she has always been very comfy in this.
I have just been looking into a new booster for my second child and found that the Britax Parkway SG and SGL have a crotch strap through which the lap belt is threaded to prevent your child from sliding or scrunching themselves down below the lap belt. I think its a nice safety feature that the Moneterey doesn't have, but this has never, ever been an issue for me. Perhaps it is because of the angle of the Monterey seat. They advertise it as being for this purpose. Or perhaps it is because I threaten my daughter when she moves herself into an improper position in her seat. Either, way, I've decided to go with the Monterey again because the Parkway has quite a few complaints about the shoulder belt guide and some about the cupholders.
My only frustration is what I mentioned earlier about how difficult it can be to insert the shoulder belt into the guide. My daughter pulls her seat belt out so far as she leans to the side to buckle the belt, that she engages the auto-lock on the seatbelt retractor. I don't realize this, however, until the next time we get in the car and the belt won't budge. I then have to take it out of the belt guide so that I can retract it completely and then re-insert it. Of course, this always happens when we are in a hurry, so I get frustrated. I am trying to teach her to pull it out less, but she needs a lot of slack in order to successfully buckle the belt on her own. Maybe my old car has a shorter seatbelt than newer cars, and the auto-lock engages sooner than it would in another car. I really don't see how this can be a legitimate complaint about the seat, however, since it is actually a good safety feature to prevent the seatbelt from slipping out of the guide. If there is a booster out there that has a shoulder belt guide which is quite easy to slip the belt into, but which also doens't allow it to slip out, I'd love to hear about it.