Product review (4 stars): Great for hand washables and to supplement trips to the laundry mat
I have read some bad reviews about this product and I honestly can't agree with them. Apparently, there is a cheaper imitation out there which is causing people to give this product  negative reviews. I received the genuine Wonderwash, not the generic knock-off, and that is the product I am basing this review on.
First, yes, it is made of plastic, but not the "flimsy" plastic some people claim. Of course it will break if you drop it or are extremely rough with it, just like any plastic item would, but mine has held up nicely with regular use. I have also read some reviews claiming of leaks. I have not experienced this at all, although it is very important that you make sure the lid is on tight and a vacuum seal has formed (you will feel the lid rise as you turn the vacuum knob when sealed properly). You only need a very small amount of detergent to get a nice amount of suds.
The drain valve is a nice feature which eliminates the need for heavy lifting to dump the water out.
The only thing I don't like is that there is obviously no way to spin excess water out of clothes and you will need either a spin dryer or hand wringer. Otherwise, it will take forever to dry clothes in your electric dryer.
The Wonderwash holds a decent amount of clothes for its size (about 3 shirts and a couple pairs of pants) but it obviously will not hold large items like blankets. If you plan on washing towels in it, expect an average of one large bath towel per load, or 3-4 hand towels at a time.
Definitely not a replacement for the laundry mat, but it certainly will cut down your weekly trips and save you money.