I am a lifelong adventure / point-and-click games fan, but this game actually passed me by on release. It wasn't until I purchased it for the Nintendo DS that I discovered it was originally a PC game, and bought the PC version accordingly.
In many ways, the game comes across as a 'Broken Sword' wannabe (the 'Broken Sword' series, in case you don't know, being one of the leading and most popular point-and-click franchises of the last few years); but that's no bad thing.
I wasn't sure about the game at first, but I quickly found myself immersed in the plot and its characters. There are no fussy controls to get to grips with, and a nice feature is the hotspot finding icon, saving hours of aimless pixel-hunting (something that often blights other adventure games).
One thing I must comment on is the graphics. Crisp and detailed, they really bring the game to life, and are amongst the very best that I have seen in a point-and-click game. There are some impressive cut-scene videos, too.
Many players have complained about the dialogue volume shooting up and down during the game. I did notice this, but what bugged me more was that many of the pieces of dialogue were read out of context, with emphasis on wrong words.
The game is generally quite fun, although there are some obscure puzzles. One of the most bizarre comes quite early on in the game, where you have to tape a mobile phone to a cat to record a telephone conversation! (Yes, really!). This sort of bizarre puzzle might have been amusing in a 'Monkey Island' game or suchlike, but in a game such as this, it just seemed totally out of place.
There is an overall plot to investigate, but the game is generally broken up into smaller scenarios, with you having to solve a series of puzzles and actions before you can progress to the next section of the game. I found this design to mostly work very well. The only real gripe that I have, as mentioned above, is that some of the puzzles are just too obscure - often you have no real clue what you're supposed to be doing, and even though I am a hardened adventure gamer of over 20+ years, a few times I admit to resorting to a walkthrough, just to give a me a clue what on Earth I was aiming to do in the game.
The dialogue is mostly serious, but has the odd humorous line thrown in here and there (which work with varying success). I was glad that it was nowhere near as heavy going as some point-and-click releases of recent years.
I finished the game in about a week, but that was after putting some serious hours in, and As soon as I finished it I ordered a copy of the sequel. The adventure game market has seen a sharp decline in the last few years, and what ones that have been released, I have mostly (with a couple of exceptions) found disappointing and hard to get into. But I found 'Tunguska' to be very engaging and addictive. For the cheap price I got it for, it was well worth it and I would recommend it to adventure game fans. ...Though I do feel they could have gone for a more catchy title than the generic-sounding 'Secret Files'.