I have only played numbers 4, 5 and 6 in this pack, but I can say right now that for those games alone this is already an amazing value and should be a must-buy for anyone even remotely interested in RPGs.
First, some general things about the series. The games are party-based and played from a first-person perspective. 4 and 5 are entirely turn-based, and movement is done on a grid, i.e. the party moves one square at a time, and can turn in 90 degree increments. 6 has a full 3D engine and is played in real-time but has the option to pause the action and enter turn-based combat. While paused the party cannot move but can turn and attack, shoot or cast spells at enemies. Generally the focus of the series is on exploration and combat, as others have mentioned, but also on puzzles, especially in 4 and 5. I felt that this was refreshing; while there is definitely lots of combat and loot to find, as you might expect in other RPGs, there's also quite a few brain teasers, which really stand to set the dungeons and other areas apart from one another. The series does a great job of avoiding a feeling of same-ness... yes, you are delving through many many dungeons, but each has its own theme and feels very different. Another thing I liked about the series is that to level up, your characters need experience, but then they also need to visit a trainer. At the higher levels, you will find that you are limited less by experience and more by money. Also, true the the name of the series, you will need both might and magic, with both your fighters and magic users being basically indispensable.
Of the games I played, I liked 4 and 5 the best. Set on the world of Xeen, which is a flat world floating in space, the fourth game features the "light" side of the world and the fifth game features the "dark" side. When both games are installed, they merge into one giant game, allowing the party to move between both sides of the world. The fifth game is definitely the "higher level" one, so players will likely tackle the "light" side challenges first, mostly, but the ability to move in between is really awesome. Also, when both are installed at once, there is an extra "third" ending added on after one has finished the main plots of both of the individual games. I also liked the turn-based nature of the game, which allowed combat to be tactical without being overly so. And the many puzzles in these games can get downright tough. I am slightly ashamed to say that I played most of them with the help of the hintbooks (I was younger then and some of the puzzles were a bit overwhelming), and if you need some hints those are included as extras in the GOG version! Fantastic. Also the world and its inhabitants were really quite pretty, in an old-school way. Colorful sprites, a huge variety of enemies and locations which are very distinct, tons of extra, optional dungeons and locations to visit for fun, and overall just a huge amount of content.
The sixth game featured a fully 3D engine, which was nice, and some of the gameplay occurred in real time. Fortunately, being able to pause combat and fight turn-based allowed tactical fighting when necessary. This was the first game in the series to have a crossover plot-wise with the Heroes of Might and Magic series, being set on Enroth rather than Xeen. The game world was set up in zones this time, generally centered around the various cities in the game. Travel between zones was kind of like warping (i.e. "It will take 5 days to travel to [city]. Do you wish to travel? Y/N"). This game retained the uniqueness of the locations and dungeons, but had fewer puzzles than its predecessors, although they were definitely still present. Overall, I thought it was a very good game, but not quite a great game, due to a few annoyances. First, there are LOTS of enemies, especially in the outdoor areas. When trying to explore around the towns, your party is greeted by hordes and hordes of similar baddies, and it frankly gets a little boring hacking through them all. There are some spells that are only used outdoors that can clear large groups of enemies, but these are hard to aim correctly and you can't get them until a little bit into the game. The dungeons are a lot better, with a more reasonable number of enemies and some very interesting and unique level design. The fully 3D engine really shines in the dungeon design, allowing a lot of vertical structure that is used in interesting ways. Another annoying bit was that your characters need to find trainers in order to raise their skills and abilities, and often all you are told is which city the trainer is in. This means searching every single house in the (often quite large) cities to find the trainer, with no way to note down which house they live in. I also didn't really like the endgame, when your party gets rather overpowered and has to mow down dragons and other really tough baddies for a while before you actually reach the end. I think the best part of 6 was the midgame. In the beginning the hordes of enemies are very repetitive, boring, and actually tough for your low-level party that doesn't have many strong spells to take out groups of enemies. In the end you're really powerful, there's no more challenge, and things drag out a little. But in the middle, things are really fun; the loot is starting to get interesting, your characters' skills are advancing and starting to set them apart from one another, and you're getting competent without being super-powered. I should note that the game is generally on the hard side, but it's never truly unfair. I liked the level of difficulty but not the repetitiveness of the combat.
I don't want to sound too negative though; even with these flaws the sixth game is a lot of fun. I think that games 4 and 5 are easily worth twice the price of this whole pack, plus you get the three prequels, the Sword of Xeen expansion (which I've never played... looking forward to that!) and the slightly-more-modern 6 along with it? A no-brainer.
Oh, and the sixth game had some pretty good music too.