As I am a chippie who carries out a lot of tiling and floor fitting, ( I sub to Floors2go) I have the three saws previously mentioned in the above posts. My thoughts.
Roberts Jam Saw
As it says on the tin, this saw is a jamb saw and not designed cut doors in situ (although mine is about 4 years old, there may be a newer model on the market). What I dont like about this saw is the height adjustment is not very accurate, two locking screws and which can be slightly offset when tightening, to set saw height. When cutting away architrave and door linning the saw has a small base plate and follows the contours of the floor so sometimes leaving an uneven cut, I also find the saw is aggresive and can splinter the architraves. Although more expensive to run due to blade costs, the Fien does a better job for this application and you can get into tight corners with the Fien unlike the Roberts. And you get a better cut with the Fien laying you flooring upto door jam/arch with the relevant packer under
tile/flooring and cutting away.
Karlton Door Trimmer
I bought this saw as I was fitting wooden flooring to a
community bungalow for disabled people. I was laying wooden flooring for a 27m2 hallway with 17 1hour fire doors with concealed self closers, I hate removing these doors on my own. What I found with this saw is the minimun cut is 10mm? which I find is to much when I only want to take 6mm off the door, I also found that the saw splintered the veneer of the doors, so I wasn't that happy with the finished cut.
The only time I use this saw now, is when I am fitting flooring to old houses which have original doors with leaded glass and the hinges are heavily painted and the screws difficult to remove, so as to not damage the frame, door or glass I cut the door in situ. Allthough there is a large baseplate, the saw will follow the conours of the floor, I know you can put a piece of ply for the saw to sit on to help flatten floor, you then have the problem of taking to much of the door.
Festool TS 55
Although I have a Skil and Makita circular saw, after reading some of the comments on the kitchen forums I am a member of, regarding the quality of the cut with these saws with the giude rails, I thought i would invest in one.
The first job I used the saw was the fitting of a front framed bedroom, the job was for two large bedrooms. As these bedrooms are custom made floor to ceiling jobs you are cutting a lot of MFC ( Melamine faced chipboard). Previoulsy with the Makita or Skil saws you would cut the MFC over size and plane to size, due to the chipping of MFC.
As the Festool has a built in splinter guard and the guide rail/track helps to reduce splinters the MFC did not need to be planed to remove chips, only used planer to scribe panels to walls/ceiling. The time I saved on this job by using the Festool probably payed for itself
As you have the guide rail /track with these saws you mark your piece to be cut, lay guide rail/track to pencil marks and cut away.
The benefits of this, is that you are not using the guide rail of conventional circular saw or clamping spirit levels to pieces to be cut and offsetting the blade distance of saws etc. Allowing you to scribe your cuts much quicker, eg cutting a door down from 10mm > 6mm across its width.
In my opinion if you need a tool to cut doors down, sheet material, plinths etc I would get the Festool. I believe that Dewalt have brought out a similar saw which you can clamp the guide rail/track to door and cut in situ, and I think you would get more use out of a Fien compared to a Jam Saw.
Sorry if the post is long winded, and as I have the all the above mentioned tools in my post, I hope my opinion helps if you are considering a new tool purchase.