Focus C-Max CVT automatics are diesel, rare for a very good reason and to be avoided at all cost. The best summary is HJ's comment ("what's bad"):

BY 2010, FAILURES OF Ford/ZF CF23 CVTs WITH 1.6TDCIS HAD REACHED EPIDEMIC PROPORTIONS and the cost of replacement at around £5,000 is greater than the value of the car. For that reason DO NOT BUY A C-MAX .,6TDCI CVT AUTO.

For gory details see http://www.honestjohn.co.uk/forum/post/index.htm?t=81143

Perhaps the best way to understand what a sequential gearbox is like is to think of a motorcycle, you have a lever that moves up and down and all you can do is change up or down from one gear to the next in range. A sequential auto box works in a similar way and is the way that older style automatics work. The auto box is connected to the engine by a torque convertor which is effectively two dishes that move independently of each other but has oil which is spun by the dish connected to the engine which in turn spins the dish connected to the gearbox. As the speed increases the higher hearing is automatically selected. There is nothing wrong with these older systems but they tend not to be as fuel efficient as those below.

A CVT gearbox works in a totally different way. Very basically there are two shafts, one connected to the engine and one to the final drive. Each shaft has a pulley with sliding flanges which are infintely variable, a steel belt or chain sits in these pulleys and transmits the drive from the engine to the final drive. Toyota have used this type for some time and there are a few other manufacturers using them too.

There is then the automated clutch gearbox such as fitted to Peugeot/Citroen cars. These simply use electronic and/or hydraulic systems to depress what is essentially a manual style clutch. Some of these systems have poor reviews but equally others think they are good. Suffice to say they are have a different driving experience to convential autos.

You then have what manufacurers sometimes call automatics but in fact are far from it, they are dual clutch gearboxes such as the VW DSG and the Ford/Volvo Power Shift. Basically there are again two shafts with for example gears 1,3 and 5 on one shaft and gears 2, 4 and 6 on the other. Each gear shaft has its own clutch and elecronics preselects the next gear in sequence. These can work as if it were an automatic , you just select drive and it does the rest or you can select which gear via the lever or paddle shifts on the steering column (do not confuse this with a tiptronic box which seems to work in the same way but is more akin to a proper automatic). The downside of the dual clutch box is that most if not all also have a Dual Mass Flywheel (DMF) which are, in a lot of cases, a weak point in the drivetrain as a lot have been failing at fairly low mileages. The cost of replacing a DMF is likely to exceed £1500.

The big downside of the modern auto box whether its a Tiptroic, CVT or DSG is the cost of replacement which is often aroung the £5000 mark.

Driving experience and whether you like one type or another is down to the individuals taste. As with almost every modern car there is little scope for the average owner or DIY mechanic to repair. All these modern automatic style boxes are very complicated and will need specialist repairers if you have problems. In most cases even the main dealers do not carry out their own repairs to these systems and simply offer an exchange system.

The best advice is to research the type fitted to the car you are interesed in to see the downsides and then give the car a thorough test drive as they all have their idosyncratic vices. Clearly the older the car and the more mileage covered the more problems you are likely to have. Happy hunting.