I only buy when it dips to £50ish quid, it's served me quite well but I did originally buy 5BTC for $20, gambled 4 of them and forgot about it for 2 years. Sold for $255 rebought 3 for $67 each after the crash, cashed some out at over $100 and bought more back at $87. The only thing I've bought with them is a couple of solid gold coins, 2 graphics cards and the steam humble bundle for the equivalent of $2.
Mt Gox may have manipulated the price in the past but now that the USAs regulatory bodies are interested and subpoenaing everyone all the exchanges are going to have to be legitimate or shut down. Also, for the owners to manipulate the price the exchanges would have to have lots of money to throw away to arbitraging bots making a profit off differing prices between exchanges.
As for mining:
Mine Litecoin, convert to BTC and end up with triple the BTC, compared to mining BTC directly.
http://coinchoose.com/ is good for checking profitability of coins, although the less well heard of ones rarely stay highly profitable for long, look at the 7day average or graphs at the bottom to get a true idea. I stick to litecoin and the occasional day once a week of something that seems low on exchanges, takes 20 mins to decide and change, 30 seconds to change back after. Export from pool to an exchange straight away, set sell order at the same price as the last small peak or slightly lower depending on how impatient you are.
Also you need a pool if you want a payout whilst you're alive unless you have many thousands of pounds worth of hardware.
Free electric is nice, if you have free power and a spare PCI-E slot or two get some 5830/5850/5970 cards off ebay and pay no more than what you expect them to mine in 3/4 months. Make sure you have a powerful enough power supply though. I got two 5830s for £75 after breaking my 7970 and getting a full refund, bought these cards with a small amount of the LTC mined by the 7970 and have an almost equivalent hashrate now that I'm using my overclocked 4GHz i3 CPU too. I have had to shove an extra power supply into the case to power it all though!
Running it with the case upside down cause I took off the original fan on each card off and replaced each with two glue gunned on. Much quieter and cooler overclocked silly. With the PC the right way up the heatsinks get hot enough to melt the glue and the fans fall off though! Better airflow upsidedown anyway.
If you have an AMD graphics card and cheap/fixed electric:
http://litecoin.info/Mining_Hardware_Comparison for settings and expected hashrates, plug that into a litecoin mining calculator, get cgminer, find a pool and away you go.