A lot of people here will be quick to tell you that the Mr Beer system is crap and that you should upgrade. I have seen some posts here from other Mr Beer users who have seemed satisfied with its results. However, it does seem that most of them (myself included) have ultimately upgraded to a more standard system. I did so even before my first batch was complete, simply because I found a guy at work that was offering a great deal on his equipment. But I think most do so because they find that they enjoy the additional effort and control that comes with the more complicated system.
But, I wouldn't necessairly give up on the Mr Beer kit just yet. IMO, it has its advantages, but also some disadvantages.
The advantages are simplicity and small batch sizes. Even though I have upgraded to a more standard set of equipment, until I'm a bit more familiar with the process and the effect of certain changes, I'm sticking to the smaller batch sizes (~2.5 gal) and the Mr. Beer ingredients. This allows me to use the Mr Beer keg as a bottling bucket and bottle about 24 12-oz bottles per batch. With the amount I travel and the small amount I drink at a time, this should work out well.
The disadvantages are:
1) Booster. The use of the Booster will result in a cider-like flavor. You have already avoided that by going all-malt. But don't feel like you have to stick to the Mr Beer brand of unhopped liquid malt extract. Buy some dry malt extract and experiment with the amount. A large part of the appeal to home brewing is experimenting and finding the right mix that pleases you.
2) No provision for secondary fermenting. Secondary fermentation will clarify the beer and refine the flavor. But it's not necessary if you're happy with the result from primary fermentation alone. If you want to try this, it could probably be done easily enough by just purchasing a vessel for secondary fermentation and some siphoning equipment. You don't necessairly have to purchase an entire kit.
Bottom line - if you're mostly happy with the Mr Beer, you can stick with it. If you want a higher alcohol brew, experiment with more unhopped extract (dry or liquid). Also check out the Mr Beer website for some more advanced recipies. Maybe try using some different strains of yeast than those offered by Mr Beer. Don't think you have to totally abandon the Mr Beer. If you think you'll like a little more control over the process and would enjoy the effort and results of secondary fermentation, it may be worth it to upgrade to a more standard system.