We're five months into our Dungeons & Dragons Basic / Expert campaign, and it has been a blast. With 92 hours logged so far, we've managed to creep up the experience scale to right about 6th level. We've enjoyed how simple the game is, but that only lasted so long. Now, our players are hungry for those forgotten bits of complexity that gives Advanced and later versions their staying power and are beginning to branch out in ways they haven't before.
At 4th level, I allowed the Expert book to be used freely by the players. It starts out with a treatise on higher level characters, up to 14th level. However, the gains characters make each level are dispersed across the levels. This results in massive boosts to certain classes at certain levels, while other levels don't do anything but give you a Hit Die.
More equipment is added, most notably sailing vessels. This has been a focal point of our campaign so far, as it's based around the Sea of Dread. Some other basic land transport information is given, but it's all "basic" stuff here.
Of course, more spells. There aren't very many spells per level, which is great for new players or players using spellcasters for the first time. It's easy to wrap your head around things and get the hang of the most basic of spells you may encounter in your adventuring career.
Basically, everything in the game is fleshed out to a higher degree. There are more spells, monsters and treasures. There is more info on combat, and how you can engage in combat. Notable additions there are mounted, naval and aerial combat rules.
However, it's the wilderness rules that are the star of the show. Not really much here in the way of text, but the way it opens the game up from just the basic rulebook is felt by all involved immediately. Now you can travel from town to town, from town to dungeon, from dungeon to dungeon or whatever. And also...ships.
Non-player characters are given more attention as well...especially the different kinds of specialist PC's can hire. You'll need sailors for your ships' crews, and you'll probably need an alchemist or sage at some point. Not to mention the brutal mercenary army you always wanted.
There's a bit of Mystara information in the very back as well, all centered on Karameikos. It's very basic, but this publication was the seed from which the Known World was grown. As it is, it shows the Dungeon Master how to set up an outdoor campaign setting in a traditional hex map. This is nice, since everyone knows Karameikos is full of fun times.
In conclusion, the Expert rulebook is essential to a Dungeons & Dragons game that wants to expand from Basic. However, all these rules are included in the Rules Cyclopedia. None of this is still in print, but if you fancy giving it a go it and it's Basic and Companion cohorts can be had pretty cheap secondhand.