The Intellicon 3250HW+ is a slightly more sophisticated economizer than the Beckett Heat Manager (which was designed by Intellicon) for about the same money, and is a bit more flexible when dealing with indirect-fired hot water heaters (which is also a good idea for increasing net-efficiency.) But either will do.
But these products don't increase the efficiency so much as the reduce the severe hit you take in efficiency with high-mass boilers that are oversized for the loads. Far more important to both long-term reliability is sizing the boiler correctly to the load. A 150KBTU/hr boiler serving a 4-zone house with a 30KBTU/hr design day load with low-mass fin-tube heat emitters is going to be a short-cycling piece of misery, even with a Beckett or Intellicon. A 50KBTU boiler on the same house would come close to hitting it's AFUE numbers even without the economizer, but would run fewer & longer burn cycles with the economizer control, reducing wear & tear, buying a percent or two reduction in heating fuel if it's cut up into zones, but not too much if it's a single zone.
Most high-mass boilers installed in my area are 3x or more oversized for the actual loads. Using fuel use against weather data with the prior boiler is a more reliable way to size a new boiler than any Manual-J type heat loss calc, since it's an actual measurement, not a best-guess on construction methods & air infiltration rates. Even carefully done Manual-Js hit to the high side, often 25-30% to the high side of measured-reality.
Depending on how big the prior boiler is/was dropping to a right-sized boiler may require a narrower flue liner. In some instances it's cheaper/better to go with a right-sized power-vented 85-88% unit and side-vent it. The lower limit to being able to chimney vent the thing is ~83-84%, above which the risk of condensation in the chimney and back-drafting due to low an exhaust temps becomes an issue, assuming a right-sized flue. If you're replacing a 200kbtu/hr 80% 6-8plate behemoth with a 2-3 plate 50-60K unit you won't be able to use the same chimney (and if you have a hot water heater venting into the same flue you'll have to do something about that too.)