I used one of the original Antec Kuhler 620's - the smaller 120mm version - for a while on my 2500k. It worked well once I got some improved fans and ran push+pull. It kept the temps down ok with a 4.6ghz Overclock too, though it was a little noisier than I'd like.
I later moved on to "proper" water cooling, with a huge external rad, low-rpm (silent) fans, a decent D5 pump. A better overall solution for cooling now multiple devices, including two GPU's (first 570's and more recently 680's) - however, due to my quest for SILENCE (really, my laptop is noisier) cooling was better than the AIO 120mm unit of course, but not staggeringly so.
One thing I did notice on the smaller unit though was basically "heat soak". In essence, the unit would displace the heat from an overclocked CPU working hard quite happily during benching sessions - we're talking OCCT + AVX here, so pushing things hard - but, during a much longer, but less stressful gaming session, temps would start to rise.
Basically, the smaller units are fine but, IMO, they NEED push+pull to be most effective and they need a slightly more agressive fan profile, which gives more noise when pushing things. The smaller volume of liquid vs. "proper" water cooling can become saturated quite quickly during a gaming session. Of course, the warmer the coolant, the less efficient the heat exchange from CPU to liquid is, so the hotter the CPU can get. Like I said though, I only really saw this heat build up in longer gaming sessions (ok, that's pretty much ALL of them!) in a demanding title.
If you're running a stock system, then I really don't think you'd have any worries, as these coolers were comparable to "good" air coolers, yet ran a little quieter.
A tip for you, and what I'd do now if I was using an AIO. I'd NOT go push+pull, I'd use ONE set of fans in PUSH, but I'd use a shroud. These have the effect of MUCH reducing turbulance, so the air pressure is cleaner and quieter. Indeed, my full custom system uses shrouds, which means the fan blades aren't spinning just a couple of millimetres over the rad fins, which is a lot quieter.
Anyway, I rambled lol.
Scoob.