Heat Exchanger ( aka E61 with gicar box) espresso machines

Pros:

Good looking E61 grouphead

Energy efficient 

One boiler instead of two. Smaller footprint 

allows users to steam milk and pull shots simultaneously or back-to-back immediately.

Good choice for price conscious( $1500 to 2500 USD),  less expensive than Saturated groups 

Cons:

Affected by water inlet temperature, esp in cold weather in freezing winter

Affected by boiler pressure 

brewing temperature range  ( usu +/- 2 to 3 degress c), hard to control to narrower range, which is seen as a disadvantage for many baristas.

Recent developments:

In recent years we have seen lots of HX machines emerge with either PIDs like on the Profitec Pro 500 and the ECM Technika V,

 or a 3-position switch that controls the boiler's temperature( eg 90, 92, 94 degrees c) giving you a low, medium, and high setting like the Profitec 400, the ECM Mechanika VI slim and the Lelit MaraX PL62x( another Jim can achieve 2 degree spread).

Lelit MaraX has 2 thermal sensors (NTC), one at top of boiler, one at bottom (coffee mode sensor), uses gicar box (likely based on PIC16F microcontroller)for PID and other features, shot timer diy github

Challenge:

As the price advantage of  non-E61 Dual boiler vs HX  is diminishing, it's interesting to see how it will evolve. Anyways it is no easy way to cost down the 4 kg of copper brass ( E61 grouphead), sometimes  such non-E61 DB is even lower by 250usd

Reference 

Espresso outlet 

vs Breville dual boiler