I used Alpari for a long time with a demo account strictly for MT4 charting. Unless they offer a different data feed for live accounts than for demo accounts, there were quite a few frustrating lags. Lags during news volatility is understandable, but during regular breakouts is really annoying. Luckily I was doing the actual trading with Oanda so it wasn't my only source for live price, but the disruption in the price feed was frequent enough and had a long enough recovery time that it was pretty noticeable. (I actually previously wrote a blog post about it in more detail: whipplebox.com/?p=247) But if you're swing trading or holding for days at a time, maybe it's not that big a deal.
Not that I'm endorsing Oanda, because they have their frustrations, too, after all I did transfer my capital from them to a different (futures) broker. But that's another topic (and blog post).