I've been trading with LMAX for several years, and really dont have anything to complain about.
The API is good, and I have not had any issues that I can remember. Over the last few months I have also traded via the MT4 bridge. When comparing the tick data on the MT4 and API feed, there is a noticeable different, and as a result I don't reference lower time frame charts on MT4.
Fills on the MT4 do seem to lag slightly during times of lower liquidity, which is normal but not something I noticed via the API orders.