I just installed one. I think it's very good indeed. But allow a lot of time for the build. It's really a collection of Swiftech parts rather than a simple kit. The performance is very good though. I have the rad attached to the back of my case. A few points:
1) The ramsinks for the GPU are not great. They work fine but the glue on mine just wouldn't hold them on and I don't fancy epoxying them on.
2) The PCI slot blanking plate adapter to allow for the tubing to exit the case constricts the tubing a little. It's not too much and probably fine but I haven't yet put it on (just left an empty PCI plate).
3) On my system the chipset waterblock won't fit because it fouls the graphics card. So again I didn't fit it.
But overall I'm extremely pleased. At full speed the fans are noisey but you really don't need them on full speed. Mine are very quiet as they are turned right down. I run mine from a fan controller (not supplied - although they do supply 12v, 7v and 5v adapters).
I run a dual core Opteron 165 overclocked to around 2.5ghz. It's also cooling my 7800 GTX 512 (@611mhz core - up from a stock 580). But as mentioned it is not cooling the chipset so that will help my temps. At idle my CPU is 28 to 31 degrees depending on ambient temps. Under load it's around 35 degrees, possibly a little higher (not seen it above that yet but I have only just installed it so not really tried too much with it yet). The beauty of water is not the idle temp but the temp under load. It can soak up much more heat. If I recall, my GPU temp is in the 40's or 50's now but I've no idea what it was on air because I didn't look.
Another slight downside is that, having a small case (ATCS 111) the pump is where the HDD's were in the 3.5" bays (now moved into the 5.25" bays to allow room). This fits fine but it blocks my two front 80mm fans. This, combined with the empty PCI blanking plate means the airflow in my case isn't as good as it was. But conversely most of the heat is being expelled outside the case by the rad so it doesn't need to be. I am, though, getting a warm chipset because of it. This is not an issue specific to the Swiftech Apex and would happen for most watercooling. But it's something to bear in mind - you still need airflow in the case.
Would I buy this kit again? Yes I would. I had not watercooled before and although I'm very experienced with PC's, I was a n00b at this. There are lots of small parts and it takes some planning to get the tubing right too. However, if I were to buy again I probably wouldn't get the Ultra+ version. The ramsink glue is weak and the chipset waterblock can't be used on my system due to obstructing the gfx card (DFI SLI DR with a large gfx card). So I'd probably get the non-plus version and add a suitable GPU and chipset block.
But for a first watercool build I've very happy with it and don't regret it.
EDIT: It didn't help me clock my CPU further but I'm probably at the limit of it anyway. It did help push my GPU a little further. Funny enough I also had an XP120 HSF and found the same - good idle temps but they raised quickly under load. My case cooling was ok but not brilliant and I was easily getting into the 60's under load - that's why I went with water.
EDIT2: Get some distilled water from a nearby Halfords and also a PSU tester is handy for filling and leak testing.