Playing Mean Streets was a nice experience. At the beginning I started to jot down some info, because you definitely need to, otherwise you won't be able to go very far. But at some point I realised the half page I had wouldn't be enough, there was so much stuff to keep track off, I had to take a full sheet.
And that was fun ! It really felt like I was interrogating the people I met, asking them about this or that, getting a feeling of accomplishment when someone gave me something to work with, and overall acting like a P.I. would. The story was compelling, the characters looked real, even if some had two head or just one eye in the middle of the head...
The only problems with the game are easy to identify : the travelling and the fights.
To go from one place to the other, you have to actually fly your speeder in San Francisco, L.A., etc., and it gets annoying pretty quick. As for the fights, it's just you advancing towards enemies with your gun. They also have guns, fire a lot, you have to duck, but you can't shoot while ducking, so it takes a lot of time, and getting killed thus is really frustrating.
But on the whole, these two flaws are easily forgotten when you complete the game. You really feel like you've solved the case, using only your brain. OK, your gun too.
Martian Memorandum took out the boring parts to keep the gameplay centred around the story, and that's an improvement. Although, an other flaw arises that didn't existed in the first game : you can die by just trying something, just like in some old Sierra games. And that's really irritating each time it happens. But that's nothing a save point cannot undo, so it's not that big an issue.
This second instalment also changes the gameplay quite a lot. Whereas in Mean Streets the main focus was talking to people, here we have a point-and-click gameplay, with less emphasis on interrogations, but you still get a lot of discussions and informations from the colourful characters that people Earth and Mars.
Another big difference is that you won't have to note down every piece of information you get or travel "by hand", the game will take care of all that itself. You can either consider that an improvement or a lack of immersion, but it is well-executed and grants a change of pace, so you it's not all bad.
In conclusion, these two games are definitely worth your time if you like adventure games, or want to feel what it's like to be a private eye straight out of a film noir. What's more, they're not really hard and play smoothly, so you won't be pulling your hair out in despair, but rather will enjoy the ride.
And don't forget what a wise man once said : Save early, save often !