Hiya,
We have fin rads on our boat, with the before mentioned Aldi, they both work very well too.
A few things to remember about both, first the Aldi; they get a lot of stick for using a lot of gas, indeed our can get through a 13kg bottle in three or so days when its cold, so thats a bout 26kg a week, but £40 (ouch). our stove burns 50kg of fuel a week, makes a lot of mess and needs looking after to keep it going, but despite using twice as much fuel as the Aldi, costs around £18 a week to run. We run the stove 24/7 in the winter and use the Aldi as back up, and to warm the boat through in the mornings.
Now the Fin rads; We have fitted Rads from the Aquafax range, which have two copper pipes running through aluminium fins, with the Aldi in the middle of the boat the hot coolant flowing to the front and back down the top pipe, T'ing off for a towel radiator in the bathroom, and the calorifier, and using Aldi's own rubber U bends and corners (Some with bleed taps), returning the cold coolant along the bottom pipe. I have made cavities behind all the cupboards in the galley, and around the back steps, to keep any damp problems to a minimum. Because the fin rads work by the convection of air, you need to allow for this by making it as easy for the air to flow as possible, and because the Aldi has only a small, low consumption pump, it's a good idea to make the coolant flow as easly too, which is where the Aldi bends come in. In summer, we turn the heating off to the rear and front of the boat using Hep 2o gate valves, leaving just the calorifier and towel radiator connected, the Aldi soon heats these up and starts going on and off. With the valves open the Aldi will quite happily heat away, the Fin Rads managing to dump most of the heat, any heat which is returned to the boiler I think is from the Towel radiator and calorifier. I would say you do need a lot of fin rad to heat the boat, probably the length of the inside of the boat if you can manage it say 40' (behind cupboards and the like), also, fin rads do not store heat as they contain little water, so when the boiler is off, the heating quickly goes.
I would not be without either the stove, or the Aldi boiler, and the Finrads do help keep the boat both warm and dry. I would not swap the Aldi for a diesel unit ever, why? well no noise, no fumes, low power consumption and good reliability.
Well that's my point of view, hope it helps.
Thanks
Gordon