First off, make sure that you look at the local, state and regional regulations regarding OWBs. Many states and regions restrict their use. For example, WA state has a ban on all OWBs, period. In Oregon you now have to get an EPA certified one (not just OWBs, it applies to all wood burning appliances). Almost all cities have some restrictions on OWBs.
Aside from that, we were very happy with my ex's classic Central Boiler (CB) system that I designed and installed. It is still in operation after 6 years of steady use. She lives in a rural area of the Coast Range in Oregon. The boiler was a retrofit for an electric hydronic floor heating system, and an electric/solar hot water system. Total cost was under $10,000, and the payback was last year (after 5 years of use). The benefits were also having a LOT warmer house in winter, so the real payback was probably after 3 years of use (if you compared the price of electric heat set to the temperatures we set the house at with the OWB). She has 100+ acres of trees, so the firewood was free. One drawback of a classic earlier style OWB is that they eat a lot of wood. We burned 8 to 10 cords a year in a mild winter climate area. However, the house is large and has tons of windows and skylights leaking heat like a sieve.
New EPA certified OWBs are going to run more money. They are basically cleaner burning outdoor wood gassifiers with a smaller boiler and water storage system as compared to a Tarn or Greenwood indoor gassifier system that requires a large water storage tank. CB has duel fuel OWBs, by the way, and you could install a newer EPA OWB that has a propane backup burner in it. Not to defeat the purpose of the OWB, but a rather large drawback to an OWB is that you have to load them with wood every day, typically twice a day, in order for them to work. If you have a backup propane heating system, you can go on vacation in the winter and keep the house from freezing.
I looked into many OWB companies before selecting Central Boiler. Many OWB companies have come and gone, and many have cheap or poorly designed systems. CB has good designed systems, they build them with good steel and good welds, and they last a long time. They make good on their warantees, and they have been around for a long time. They also have excellent technical support, and they speak English on the phone. They helped me design the system and size the flat plate Hx's correctly. I would say that the sizes were perfect for the application. If anything, I would say that the OWB was larger than what we needed, but that was the smallest one that they sold at that time. They still sell the classic systems, and in my book they are well built, they work well, and they hold up over time. They are also simple open/non-pressurized systems, and as such they are safe. I had several overfiring situations as a result of a faulty contol unit. During the few times that the system boiled over, it just did that. It boiled over and all the heat went with the steam out the vent, and that was it. No meltdown, no exploding pressure tanks, no flame outs, no damage or burned anything. I just topped the system up with water and restarted the boiler and it ran fine. I replaced the control unit under warantee, and they gave me another gallon of corrosion resitant stuff to add to the boiler water. All for free, all under warantee. CB sells the best corrosion resistant water addative that I could find as well. I would put one in my house here, but the new laws prevent me from installing a classic system in Oregon.
Oh, and no, they do not smoke much. No more smoke than what I get from my older pre EPA wood stove here, anyway. That befuddled me for a year after turning on the OWB. Where was all the smoke that was supposed to pour out of the OWB that the 'ALL OWB's ARE PURE EVIL' people rant and rave about in an endless steam? It was just not there. Burning 24/7, I could smell smoke now and then if the wind was right and I was outside. However, there was never any smoke or smell of smoke inside the house, ever. Most of the time there was no sign of smoke from the stack either. There was a brief puff of smoke when the damper opened for all of about a minute, and then that wafted away. In off mode and burn mode, narry any smoke was visible from the stack. But the legal system chooses to believe that all OWBs are evil, and so my first sentence in this post.