I also started with Mr. Beer and brewed every kit I could think of with them in my first year. Last year I went all grain and I am now building a two tier electric RIMS system (yes, the hobby is that addictive). I agree that Mr. Beer is a great way to learn the ropes and see if you want to invest more time and energy (and money) into the hobby.
A few recommendations for the best Mr. Beer possible:
• Read John Palmer’s How To Brew introduction and extract chapters first: http://www.howtobrew.com/
• Remember clean everything first, then sanitize ANYTHING your finished wort will come into contact with.
• The One-step oxygen cleaner/sanitizer is OK, but I would suggest a small bottle of iodophor OR Starsan sanitizer. ** Use distilled water to mix an ideal solution and store some in a spray bottle**
• Don’t use the "Booster". Use more dried or liquid malt extract (DME or LME) to increase fermentables and alcohol. Either buy the un-hopped LME from Mr. Beer or, better yet, your Local Home Brew Shop (LHBS). If you get more that 20% of your alcohol from booster/sugar/honey your beer will taste cidery.
• Ditch the no-name brewer’s yeast under the caps and invest the $3 on a pack of Safale or $6 on a vial of liquid yeast. It is a noticeable difference especially if you are going for a certain style
• Rehydrate dry yeast in boiled and chilled water before pitching into your wort
• Keep fermentation at a constant temperature (within 2 degrees of target temp for your yeast) and free from light
Brewing is like cooking; quality ingredients and thoughtful processes will lead to a better end product. Just experiment, have fun, and remember that there are no mistakes… just more and less drinkable experiments.