I happen to have the WeDo kit and I just got an extra USB hub and motion sensor for it. In general, WeDo feels like Mindstorms little brother - and by little I mean much more restricted and much less intuitive. The main cons are:
- limited mobility, unless you enjoy running around with your laptop in hand
- limited precision, because all the motors are controlled not by counting rotations but by a simple timeout function (I've tried to build a simple 2D printed with WeDo kit and it failed even to draw a square)
- limited use in general, as the kit comes with a single USB hub that can control up to two devices at once (the WeDo software can control 3 hubs at once so you can plug up to 6 devices, but the hubs don't come cheap)
- restrictive programming environment, because instead of a genuine programming language you get a software that allows you to create algorithms by putting together a series of blocks, each block representing a single instruction (such as e.g. run motor for a given time, run motor at a given speed, stop motor etc.)
- THE PRICE!!!
As for the "programming", I have to admit that despite dealing with advanced php, js & jquery almost every day at my job, I had serious problems getting the grasp of the WeDo stuff. It will feel weird for anyone who had something to do with programming. The 'instruction blocks' I've mentioned can be combined in various configurations, but are nowhere as elastic as e.g. basic php is. For example, you can create a simple conditional statement such as if/then, but forget about if/then/else. On the interesting side, the WeDo provides 10 levels of motors speed (more than with the recent dual-dial PF handset) and can run multiple algorithms at once (namely, the WeDo algorithms can send 'messages' and be initiated on a specific type of message, thus providing the possibility for a 'mother-algorithm' to control a number of 'child-algorithms).
There are two types of sensors that come with the WeDo kit: a motion sensor and a tilt sensor. The motion sensor is in fact more like a distance sensor: it detects object at distances between 15cm and 1 cm and returns a 0-10 points output indicating the distance. The tilt sensor is supposed to detect forward/backward/left/right tilts. Unfortunately, its accuracy is disastrous. It can work very well determining the tilt of a baseball bat you intend to swing, but forget about e.g. putting it into a vehicle to detect slopes.
It feels like there was plenty of options that could be embedded into WeDo but were left aside. For instance the software automatically picks up the input from your computer's mic. But forget about WeDo and voice commands - you can only determine whether there is some sound or not, or you can play the voices you've recorded. The same thing applies to the motion sensor - it can measure the distance with a satisfactory accuracy, except no WeDo instruction makes real use of it - you can just determine whether something comes into range or not.
Long story short, having the WeDo kit in my possession for about a year I have not been able to come up with a single practical MOC based on it. On another hand, things will probably change now that I have two USB hubs and a handy little netbook to plug them in. Still, if you hesitate between WeDo and Mindstorms, Mindstorms wins in all aspects. Some enhanced programming software and new, more interesting devices could improve WeDo's potential, but I don't really see that coming.
Sir, you are insane