Road or mountain bike?
If it's mainly for road use, I don't think switching to disks would be high on my list of potential upgrades. There are some reasonable Avid cable operated systems, but even then, it would be a debatable thing to do on a cheap bike.
However, if you are a keen mountain biker who is wearing out a set of V-brake pads every weekend, or is destroying several sets of rims per year, or is getting very scared by a lack of braking efficiency down very steep hills, then it's a no brainer. Disks are the best upgrade to a mountain bike ever and in particular, Hope Minis are fantastic.
Even if you fall somewhere between the two 'extremes', it's worth remembering that, although hydraulic disks are somewhere around 200 quid per set, they can easily be serviced back to 'as new' condition so they will last you well over 10 years and it's a simple mater to switch them over to each new bike in turn. Mine are on frame number three.
So, I guess it's the same as everything bike wise, it's a balance between need, want, cost and value for money. Your call!