Reader, be forewarned: Aalto is my favorite soft-synth. If you're looking for a review with more "cons", I would check the much more professional reviews at Waveformless, MusicRadar, and elsewhere.
Going down the line:
INTERFACE: Probably as straightforward as it gets. All the controls are visible at all times. The only menus are for patch presets and for built-in tuning tables(!), of which I counted over a hundred, including African scales, Japanese, Gamelan, and several variations for Just Intonation. The control dials double as oscilloscopes so you have a visual grip on each module's activity. You can turn the animation off, ostensibly to save CPU. The action in Aalto is much like a touch-screen device: the values on a control will "jump" to whatever position you click on. I've heard complaints about this, but I take the opposite view: I wish all my software instruments behaved like this. Nowhere in Aalto's design is any attempt to "look" like a hardware synth. No fake wood veneer. No virtual dangling cables. Totally 2-D and functional. It looks sort of like a _really_ nice Max patch. It looks like the opposite of Ultrabeat, which is really what matters. How could Aalto improve here? An "undo/redo" function and more ergonomic patching.
SOUND: Different!! Finally. Unique and widely varied, but comparable to the sound of the Buchla 259, which inspired the design of Aalto's complex oscillator. That kind of beefy FM goodness. Great for percussion. Capable of stupid levels of bass. Excels at drones. To these ears the complex oscillator sounds way deeper than the two-oscillator FM synths. Plenty of waveshaping and timbre controls. Also a waveguide/delay with enough range to be used as another oscillator. At extremes the waveguide feedback can sound like a nasty (in the good way) guitar or a helicopter landing on your house.
FEATURES: Well balanced, and not excessive. Aalto takes only a few minutes to get familiar with but then has enough control to keep you busy making a unique patch every day for the rest of your life. Speaking of, I can't tell you how many HUNDREDS of patches I've made with Aalto. With a lot of other synths I've had such a bummer programming them that I've ended up buying expensive sound sets just to cut to the chase for the right sound. Aalto inspires you to dig in and roll your own. Similar satisfaction to patching up a hardware modular. I've heard more than one Modular Guy say that Aalto is their first, and last, soft-synth. It's worth checking the dev's website for their thoughts on design philosophy and the things that make good instruments deeply playable.
DOCUMENTATION: The best instruments and tools don't even NEED a user manual. You just start playing them. This is one of them. But it comes with all the documentation you would expect. Online tutorials, PDFs, etc.
PRESETS: Comes with some good ones, usable, playable, and appropriate for demonstrating the different patching possibilities, though this is one area that could use improvement. Another plus would be better patch management (prev, next, A/B compare, etc), though the developer has indicated that forthcoming updates will include a lot of new presets, and possibly the ability to search for and share patches online from within the software. The presets are cool, but, again, this is a synth that will probably have you making your own.
SUPPORT: Quick responses from the developer via email and Soundcloud. Seems like a small company owned by nice people.
VALUE: Couldn't be more satisfied. What hundred bucks?
STABILITY: Solid. Not a single crash in any host I've used: Audiomulch, Bidule, ReNoise, and Logic. Developer seems careful. Sometimes it feels like _too_ careful - the updates seem to take forever - but I would rather see a solid release later than a buggy, half-assed release sooner. Heavy on CPU - with all voices going it's about like ACE in "accurate" mode. Not a bummer if you're able to freeze or render to audio. Was able to get twelve voices going on a 2009 model MacBook Pro without glitching (in the bad way). CPU load is also supposed to improve with imminent updates.
SUMMARY: Really excited about Aalto. Check the demo. Highest recommendation.