Portable Solar Panels are they any good? And what is the minimum useful size?

The simple answer is YES.

If in a vehicle, you can not have them big enough.

Plus big is best if possible when it comes to solar panels for the maximum efficiency. By efficiency, the actual efficiency is dictated by the design of the actual semiconductor itself, I mean the bigger array as in surface area, the more chance to catch the available photons!

What about if you are hiking?

If you are on foot then you will be limited to how much you can carry and also you will need a "flat" uncrushable space to store the folded cells prior to use or some where to lash them too if strapped to a pack whilst walking.

Minimal wattage is going to be 15W but remember going bigger wattage is great but this usually means a bigger solar arrays, so that's just current physics! and this means a bigger "flat thing" to carry.

What about these folding/flexible panels?

Well the ones which can be rolled up still present as a cylindrical object to carry and in some ways is more of a PITA to carry than something flattish.

Expensive Vs Cheap??

So for example, a Goal Zero (an expensive brand?) each panel works out at approximately 16 Euros per panel; so a 24 panel array is retailing for about 400 Euros, not including shipping?

If we compare this to a 4 panel array at the same cost per cell you can find them for as little as 64 Euros or $67 USD.

Recently I reviewed a Bulgarian brand by the name of Dulotec. 

The cover material of the DULOTEC 15W array is also water resistant (ie light rain is ok) because the ballistic nylon is coated on the rear side.

Their 15W, 4 cell panel retails for approximately $53 USD! Remember all of the panels are made in China anyway and all of the construction is also Made in China too.

Do not thing that Goal Zero is not made in China.

A 5W (waste of time) panel from Goal Zero retails for about 60 Euros!, About $61 USD!, That's chucking money away and paying for a brand name.

In reality you can not do anything with a 5W panel. Maybe only trickle charge a larger power bank if you are lucky.

 You are buying from a fixed cost market when it comes to quality solar panels. A bit like coffee prices LOL!

What about for Preppers? or just for when there's a power outage? Can it charge a phone even through glass windows?

Full morning sun, although passing through slightly dirty doubly glazed windows.

Actually after 10 minutes or less the phone (iPhone 5S) was at 34% from 27%, which I thought was pretty good considering this was full morning sun coming in through doubly glazed windows. So clearly the solar panels are pretty good IMHO (I mean let's face it, that;'s what one is really paying for, the quality of the actual cells, as I mentioned above.)