If you want to make a portable WiFi hotspot using a 3G mobile-phone signal, then there are three basic ways to go about it
a) use a suitable 3G & wi-fi capable smartphone and having it act as a Wi-Fi access point (post #7). These phones aren't cheap, but if you already have one and the data contract for it, and have read what post #8 mentioned about your mobile-provider detecting you are using the phone with other devices and are satisfied you aren't breaking your contract by doing so, then that is the cheapest option.
b) use a laptop with a Mobile Internet USB dongle and enable Internet Connection Sharing. No problems here as all traffic will be allowed including from other devices. The main disadvantage is the laptop must be turned on whenever you want to access the internet from any other device. Also beware of exceeding your traffic allowance as it will probably be very expensive if you do so.
c) use something like the MiFi from '3'. This is the most interesting option if you want truly mobile internet (not relying on a laptop computer with dongle) and don't want to buy a high-end smart-phone and use it to provide a similar facility.
I checked the Mi-Fi a few days ago as this is something I'd be tempted to buy, and checked again tonight and it looks like prices have come down a bit.
The Mi-Fi itself can be bought for £50, and then there is the option of buying 1GB of data (valid for one month maximum) for £10, or 5GB (one month expiry again) for £15. There's also the option of entering an 18-month contract and paying £2 per month extra on top of the above rates (so £12 for 1GB/month, or £17 for 5GB/month) and getting the Mi-Fi for "free" (the extra £2/month buys you 20 free SMS messages which can be sent from your PC if you plug the Mi-Fi into it, in other words a worthless extra used to subsidise the cost of the Mi-Fi itself).
If you go down the Mi-Fi route (as I am tempted to do), I'd recommend choosing the PAYG option and paying the £50 up front and buying credit as needed at £10 for 1GB or £15 for 5GB, rather than the extra £36 total over 18 months and being locked into the contract. By going down the PAYG route, you can buy credit as needed (either 1GB or 5GB for the next month, or more than one 5GB if you need to use it more) or not buy any if you won't need to use it for a few weeks.
Just beware that exceeding your allowance incurs heavy costs with the Mi-Fi. Every MB over your allowance costs 10p. To put that into perspective, if you paid £10 for 1GB but ended up using 1.2GB, those extra 200MB would cost you an extra £20. If you downloaded 2GB with a 1GB allowance, you'd end up paying £110. So keep an eye on usage and buy 5GB for £15 if it looks like you'd need it.