I had never played Tex Murphy games before, aside from a few playing hours of "Under a Killing Moon" more than ten years ago. I only got Mean Streets + Martian Memorandum because gog gave us these games for free in 2009.
A few months ago I finally decided to play the first game, Mean Streets, and I absolutely loved it. It is very novel, fusing flight simulation, adventure, action (not great action, but very interesting anyway)... I loved the setting and the mood, and Tex is really an interesting character.
I even loved the flight sim sequences, that most people complain about. It's all about immersion here, and you really feel like Tex, going around San Francisco asking questions and gathering clues. In fact, you sure get very confused and lost if you dont take detailed notes on every character you find in the game and the things they say. I dare say I've spent more time working on my notes then on the game itself. It's a real nice experience.
Martian Memorandum is less varied and more focused on being an adventure game. It's a lot easier to play, because you don't have to type in your questions, the names and relevant subjects are added automatically to your options when you first read about them. But it's still a pretty hard game.
The thing I loved most about MM was the story. There is a big revelation near the end, it's really well thought out. Sadly, I got stuck in one of the many dead-ends the game presentes, but I was close to finishing the game, so I watched the final moments on YouTube. Tex is also more developed as a character, since you have dialogue options to choose, and they are usually very, very, very funny! This is an excellent adventure game, with an above average story.
To sum it up, this is an amazing package for a very low price. It's technology is very primitive by today's standard, but if you let this get in your way, then it's very weird that you are a GOG user ^_^
I proceeded to play all games in the series after these two, and they are just as great. Play them all, you'll love the series.