If you have the cash and you can get your hand on something decent it can be a money spinner. Depends on what you want to do, some buy it cash, do it up, revalue it then get a btl mortgage, some can pull most of the money back out you might leave 10% of the fully invested money in, depends on the lender and valuer really, then move onto the next one, same scenario all over again, it's all well and done as long as it's just interest only mortgage otherwise you rarely take any rental money out of it if it's a repayment one. Cheap areas backside is with letting, the monthly rent doesn't cover a proper refurb in years to come or after a asbo tenant, you can always remortgage it when you think it's worth it to pull some tax free money out of it but then again if it goes tits up you'll less likely to lose out on a substantial amount of money than like in more hyped up places. (I'm talking here about 40-50 k worth of place with a monthly generated income 350-500 before tax) it's all depends on where you buying.
Some buy it cash again then do it up and put it back on the market, what you have to watch out for is sdlt, capital gains tax, maybe incorporation etc. There's loads to consider, but if you have a good deal on the table, where the numbers stack up just go for it