Personally, I love the Frontier and did not find it difficult to install at all (if you have the latch system in your car it snaps right in). However, I have moved up from the Marathon and am very familiar with the BRitax seats (having had to remove and wash the cover multiple times during potty training).
It is heavy, so shuffling it from car to car could be inconvienient. It is pretty large, but I drive a Sienna, HOwever, I also bought one to use in my parents' Camry and had no problems with it their either.
Also, I have heard others say that it sits too low for their child in their car and the child has a hard time seeing out the window. I haven't experienced this with the Sienna or the Camry.
My 4-year old likes the cupholders (her two year old sister who is in a Marathon is jeaous) and is very comfortable in the seat.
My daughter is petite and could have stayed in the Marathon a lot longer, but the Frontier seems more like a big girl seat for her (there's a lot more room to grow). I like that we'll be able to use it for a booster too and I'll never have to buy anything else.
Britax seats get high saftey ratings, but in addition, I've just found that they FEEL solid and of high quality. The only non-Britax seat I have bought was our original Graco Snugride (which I also liked), so other than trying them out in the store, I don't have experience with other seats.
The only complaint I have is that the buckles can be pretty stiff and hard to get opened and closed. THis is good because my kids can't undo them, but bad because they can be difficult for adults too sometimes (e.g. my parents).
So, guess I fit the "love it" category!