In theory, a huge increase in speed as there are no moving parts. In practice, although they do run a bit faster, you won't see anything like the speed increase you'd imagine.
As for disadvantages, other than the price, none really...
I'd consider getting one for a laptop that is going to be used on the move, not for the speed increase though, just purely due to the lack of moving parts so shocks and knocks can't damage (destroy) them like a normal hard drive.
I wouldn't get one myself for a desktop computer (or a laptop that spends it's life on a desk)
Upgrading the hard drive is unlikely to improve the speed of the computer, other than the obvious benefit gained by reinstalling the operating system, thereby eliminating the detritus which builds up over time (unless you're REALLY careful) and can slow things down.
Its the desktop pc that I'm trying to sort out. My brother deleted loads of internet file and did some other stuff to it the other day (he's an comp engineer/networker) but I'd like him to completely sort it out at some point.
I was tinkering with one a few weeks ago on another PC.
PDQ to be honest.
I usually use Raptor drives in Raid which are pretty fast .. but this one was at least as quick.
A clean install and keeping it clean is free and will probably make a hell of a difference, spend a little of the money you save on RAM and you'll probably be sorted :)
Just spoken to my brother and he's bought me two extra memory sticks as he says every time he looks at it the memory always looks full. Hopefully this may sort some problems out.
Some finishing shortly.
If you use both for non-raid storage, that is fast 300Gigs .. less .. say 20Gb OS and programs/partition space = 270Gb of fast storage.
If you Raid them for performance .. half that.
If you learn a bit about OS's, there are for instance about 20 services in XP that do not need to run at all for day to day computing. I switch them off .. in fact they have all been off for two years.
Items that load with Windows is another one .. My TM list is only about 20 tasks long incl Firefox running .. and 4x items in tray.
Speeds up things nicely.
You're all just going to confuse tiggs (no offence meant tiggs) and my advice is....
If your computer is slow then back up everything and get your brother to do a clean install with all the latest drivers (preferably from a original Microsoft OEM CD)
If your computer is slow and running out of hard drive space then get a new, bog standard, larger hard drive and get your brother to do a clean install on that.
If you have less than 1GB of RAM then upgrade that (no need to go mad... 3GB is more than enough for most)
Keep the system clean after the install and only install what you really need.