Sylenth1 is probably the best known vsti. It is so famous and popular that you'd have thought it would have been on version 4 by now. While I won't open that can of worms here, I will say this, that fact will not affect my review. As far as I am concerned, it is what it is now and that is what you buy it for.

User interface:
I love it. Didn't like it much at first, but I appreciate it now. You can get familiar with this synth very quickly, most everything you need is available without changing any tabs. It is not too ugly. It kind of has a classic look.

Sound:
This is why it is so famous. The Sound of Sylenth1 one is brilliant. I go off this synth sometimes in favour of something more complex, or an emulation or something, but in the end whenever I do come back to it I am hit by the sound quality. It may not be able to do as much as many synths out there but what it does it does better than most synths out there.

It's sound is really snappy, more so than just about any synth I've tried. Lennar is a genious that's for sure. It's partly this sharp immediate sound that has made it such a mainstay of dance music, then of course it is also the unison. While it is not actually a "unison" mode like you get on some synths, (the oscillators are stacked) it does the job. Fat leads, lush pads, complex timbres, you got it.

Filters:
The filters are great. I will not say sublime, but they are very good. They don't sound quite analog to me, in that their character is somewhat digital and predictable, but they are excellent and you can't make them sound bad. The drive is very good, it screams more than most VA's and adding just a small amount is genuinly useful.

Effects:
They are good if you ask me, partly because they are so flexible. This part of the synth really reminds me of hardware. With some of the old hardware synths they used to give you a lot of options. Many new ones dumb these down in favour of simplicity. Perhaps reflecting the shift in the userbase. Anyway, the reverb has a stereo width control, which I use alot to create mono-reverbs which give the sound more "body". I will still apply my stereo external reverb later.

The delays, sometimes I use the internal sometimes the external, but this is also very flexible.

The phaser is very good as far as they go. It's not the best out there, but it is far from the worst.

The distortion: Great! It offers bit reduction which I love to use a lot. The foldback and overdrive sound very good. Not quite as convincing as the real thing but it's a very usable sound in a modern context.

Features:
I like the features of Sylenth1. I like that it is simple. In fact what is given is just enough complexity that you can really do a lot with it. The sheer amount of soundbanks available for this thing prove it. I love the two layer design, the fact that Lennar gives you copy and paste functions for envelopes and oscillators makes it a breeze. The mod slots give you lots of room for sound design and the destinations are good. Phase modulation works excellently.

This is where I knock one point off though, we could have had PWM, that would have made it so much more versatile, however there are ways around this, it just would have been nice to have it plain and simple. Sync & FM would have been great too. But some how it feels like it wouldn't be Sylenth1 if it had these features.

Documentation:
The manual is a great read. In it LennarDigital shows a lot of confidence in his synth. He explains how well coded it is and how good it sounds, and he is right. It's nice to see such confidence, he was clearly aiming for the professional world of musicians and studios.

Value:
This synth is what, $180 or something? It is not the cheapest synth around but so what, if you wanted something of this quality in hardware form you'd be paying much more. In fact if you buy soft synths of lesser quality you will only spend more in the long run trying to find a good sound.

Stability:
Very stable. I've never had any problems with it.

Conclusion:
While it is not for everyone, it should certainly be considered. It is not at all just a trance synth, (there really is no such thing anyway) it is versatile.

I really recommend this synth to anyone who wants a synth hardware or software, analog or digital. If you love electronic music you will probably find something to like in this synth. It is a work-horse and a go to for real solutions.