Going on the assumption that with the increases in difficulty, it probably will take 5 years to ROI on any new investments made, can someone explain how re-investing is a good idea? If the payments are getting (relatively) smaller and your investment is getting larger, how will you ever win? Unless you are calculating in £/$ somehow?
You can start to get ROI in as little as a few hours if you purchase a contract on a Saturday.
Seriously though, the cost of buying one GH gets cheaper over time, because the expected return of 1 GH decreases over time. I don't think both have decreased at the same pace, mainly because the value of a Bitcoin relative to fiat has fallen.
Use a spreadsheet and guestimate your payouts. If you don't think you can break even in under a year, then don't bother investing. The expected return after the first year is very small.
In a worst case scenario, if you buy a contract and then experience some significant difficulty increases early in the life of a contract, you run the risk of never breaking even. I know that PBmining customers who purchased contracts through April have been breaking even, but who knows what luck someone will have if they make a purchase today.
My strategy is to consider making a purchase every three months. The best time to buy is right after a difficulty increase during a period where you think difficulty might not rapidly increase. Again, run the numbers in a spreadsheet using your assumptions before you make a purchase.
Is it a good idea to reinvest all of your PBmining payouts into new contracts? If you think every contract will always break even, then yes. If not, then no.