Yep I have used them for a good few years, in the past principally for stocks but now using them largely for ETFs. Their charges are low and I would say the execution 9 times out of 10 is very good - only occasionally a bit slow for a few minutes at market open or heightened volatility etc...
Costs are OK £10 a trade for the first few trades of the quarter then £5 a trade thereafter.
Biggest negatives would be the web technology, there are no real time prices for your portfolio, it's lagged by 15(?) minutes - they used to have live prices 18 months ago but then that got turned off for whatever reason. To get around this I mirror my portfolio in Google Finance which gives me free real time pricing.
Other negative is that when buying some of the more exotic ETFs you need to 'agree' to the risks outlined in the Key Features document - however they don't appear to have uploaded all those documents so in those cases there is nothing to agree to and you'll need to call them up. However on the phone they are very straightforward and pleasant to deal with, and their mobile app is actually not bad at all.
If you decide to go with them PM me your email and we can share some free trading credits...