I had that thought a while back - the "I really want a 7 string" theory...
So, after much deliberation I saw a 7 string Epiphone Les Paul on ebay for £200 and decided I should buy it just for the cool factor. I originally thought 7 strings were a great idea, what with the fact that the 7th string could be ignored when I wanted to use it as a 6 string.
So, £200 and 9 months later and I hardly ever use it. It's just sitting and gathering dust most of the time. One reason is that the pickups are shitee (can't afford BK's), but it's really not as useul as I first thought.
The thing you need to realise is that it's like learning a whole new instrument with the different chord shapes or whatever