There are pros and cons with both types and I think you've got the main two cons of an electric shower. (by electric shower, I mean one that heats the cold water as it passes through it, and will have a rating in the region of 8 to 10 Kw, rather than the type which has an electric pump that just pressurises pre heated stored water and is often referred to as a power shower)
Electric showers can't match the flow available from a shower fed from a combi, and they generally don't look so good being surface mounted. Although I have seen some where the heater part was remote and could be hidden away, and a nice flush control panel was all that was seen in the bathroom.
As you say you have a combi, that will only heat the water you use (as opposed to heating an entire hot water cylinder) in exactly the same way as an electric shower does, so I can't see how an electric shower is saving you a fortune. I imagine an electric shower would actually cost more to run, than one fed from a combi, unless maybe you are on economy 7 or 10 and only shower in the 'wee small hours.'
Several shower manufacturers have shower selectors on their websites which will direct you to a particular type of shower that will be suitable for you and also your plumbing system. OK, it will suggest their products, but you will get an idea.
Are there advantages of an electric shower? Well the fact that they don't use quite so much water can offer a saving if you are on a water meter, and they can provide a back up hot water supply if your boiler breaks down.
