I went LLU myself (with TalkTalk) a few months ago. The main incentive is cost. BT charges a whole lot to ISPs who resell broadband using BT lines (IPStream), hence a lot of packages on BT lines are about £20.
With LLU, these costs can be much lower. So much so, that TT can afford to give me the same £20 service for £7.
If you want to keep your line with BT (you might!), then there's something coming soon to some exchanges called Wholesale Broadband Connect, or WBC. This will improve speeds to 24+meg (up from 8mb), but the cost of such packages will likely still be around £20 minimum, as it's still a BT wholesale product (resold by ISPs).
LLU is great, most people will tell you. It's BT losing absolute control over everything from the exchange to your house. It's allowing costs to go down. The downsides are that you pay a bit more to move from LLU ISP to LLU ISP, and you better make sure your new provider has a reasonable reputation for customer service; but since BT are largely shocking at CS themselves, that's not much of a risk.
Seriously, my anti-BT bias might be showing here, but I've had nothing but bad experiences of them. And incompetence too!
One side note: to move from LLU ISP to BT Ipstream ISP can be problematic, so I hear. BT will put you on a 12-month line rental contract and have been trying to charge some people £140 for the privellege of switching back. Good ol' BT.
Fortunately, there in many exhanges there are more than one LLU provider, so you don't have to go back to BT. And even if there's only Sky (Easynet), there are Easynet resellers who you may have better luck with.
thinkbroadband.co.uk (formerly ADSLguide) are a great place to mine for information.