The 840 is apparently a slight step backwards compared to the 830. There is an 840 Pro.

That said, basically any modern SSD will provide a massive improvement over a mechanical drive mainly due to the basically instant access times compared to milliseconds.

I have an 830 and I have noticed some slow down in particular on my Downloads folder recently when opening it. Not sure what's going on there and I don't really have any evidence to specifically point towards the SSD.

When I first installed (Windows 7 x64), it was lightning fast. Reboots were stupid quick but I'll also admit I'm not as careful about what I install lately, so a re-install might clear all that up.

Also when installing I had to make sure I had the right SATA drivers for my laptop. Without them performance was good but not as good as it is now.