I spent £550 total on my watercooling and case (this includes a 10% discount voucher) and at first, it was all very fun. I was running my CPU at 3.6ghz at the time on air, so I dreamt of large clocks and large benchmark scores. The first thing I noticed was what an absolute stress it was to set a loop up. The number of times I had to take it apart to reseat CPU blocks, fix a leak with my rad, change this and that was a nightmare, took me about 3 days to get it working.
After that, I was pleased with temps. Idling at room ambients (as accurate as a low TJmax can be!) and loading at ~40c. So, with my golden VID chip (1.25v!), I went in quest of 4.0ghz! After days of toiling with voltages, multipliers and termination percentages I found my chip has a natural boundary of 3.7ghz. The law of diminishing returns kicked in pretty hard and I was left at only 100mhz more. I didn't think much of that obviously, but at least I had my low temps right?
Well, yes, I had my low temps, but so what? My chip had nearly hit it's natural limit on air, as most modern CPU's will do nowadays given the tiny die fabrication processes (65nm, 45nm and perhaps 32nm?) and voltages. For a Netburst architecture running anywhere between a 1.7v - 2.0v vcore, cooling was a major issue, but it's just not a problem anymore with such small dies. Sure, you'll squeeze a little extra out, but really, is it worth the initial expenditure? Also, the low temps are great, but it doesn't mean it's giving off less heat, it simply means that the heat is being transfered more efficiently to your surrounding, IE: your room! Graphics card isn't even worth mentioning, you hit the natural core and memory clock long before temperatures are a limiting factor. Sure, you can volt mod and I have done myself. A good aftermarket cooler will handle it fine. Anyway, the part that heats up the most whilst over volting is the VRM's which even with a full cover block, aren't watercooled!
Lastly, looks and sound. Now, I must admit it was pretty to look at, especially compared to the "disgusting black box" I have at the moment (to quote my girlfriend). But, watercooling is something that appeals to people passionate about computing and the pursuit of performance. However, never, ever underestimate the complete and utter chore that is draining, taking apart, refitting and refilling a loop upon upgrading a new part (as fanatics will do regularly!). Also, factor in the cost of buying new waterblocks (especially for GPU's) and also new liquid (unless you are cheap enough to save what is drained in a bucket and re-use that). Soon you're pushing £40 - £50 on top of your original upgrade expenditure! To put it simply, upgrading is a nightmare unless you have lots of time on your hands. Don't also be fooled by thinking it's any quieter, even with a greater fin width radiator such as the Thermochill P120.3, you're still looking at 3 fans on it and then case fans on top. Obviously, if you want performance, you will be putting higher RMP fans on in a push/pull config. My current PC is just as quiet, with slightly less fans total.
So, in all, my experience is a rather negative one. I simply found the whole process expensive, time consuming and on the whole a waste of money. It was a challenge and a fun one, but going at it with the perspective of performance is a false economy. A combination of more efficient air cooling and smaller dies means the gap between air and water is a lot smaller.
If you want my opinion, invest in a nice case and a high end air cooler. You will have more money left over for upgrades that reap tangible benefit.