i'd recommend the Turnigy Accucel 8 and its quality derivatives over that Sky charger.

It's 150w which isnt so powerful but can handle 8s so can balance 2 4s packs at once, and can handle 7a charge rates - my math approximation says 150w @ 7a = APPROXIMATELY 21.5v meaning it should charge a 5s at 7a. I've never seen harnesses for charging 4x 2S packs on an 8s charger in series, but i'm sure one can be made easily and hobbyking sell harnesses for charging 3x2S in series and 2x2S in series so you could charge quite a few large 2s packs at 1c+. Unless you want to be charging 5000s at 2c+ it's very hard to beat.. getting much more powerful gets expensive quickly when power supply is taken into account.

its not overkill for tiny little packs for a micro or AAA/AA cells for TX, especially considering the series charging capabilities, and it isn't underpowered for big cars. A lot of people here in the UK use them for e-converted 1/8 Hypers because of the 8 cell capabilities. It has good reputation for reliability and accuracy, is easy to get a power supply for, is highly specced, is very cheap for what it is.. etc etc..

That's the cheap option. The 'ultimate' option is anything from the Junsi iCharger range. I think Hyperions are the best but they're super expensive and when you consider how good the iChargers are it's very hard to justify the cost of a Hyperion.

edit: i've got the Accucel 6 which is really underpowered for racing anything bigger than an 1/18 scale mini.. it's a good charger for home use though. I'm really happy with it it's a BRILLIANT charger besides being underpowered but that isnt the charger's fault that's my fault for buying too small a charger. So happy with its accuracy, efficiency, ease of use, quality etc i'm geting an 8 and i'm really hard to please with electronics. I actually like the option of having a tiny charger for small packs or when a long charge time is ok/desirable, and a larger charger for big stuff.