So far the Jerez testing has been a disaster for the Renault powered cars. That includes triple world champion Red Bull. Through 2 days of testing Renault powered cars have managed just 38 total laps and none of them have been run at anything resemebling a competitive speed.
This morning Red Bull had another problem on their initial foray on to the track (smoke, fire extinguishers, and coming back on the loader). Toro Rosso hasn't done any laps better than 1:45 (top time is 1:25) and Caterham has yet to set a time today. It looks like Lotus isn't missing out by not being at this test.
These reliability issues for Renault could make a serious impact on the start of the Formula 1 season. Not only that, but on the end of the season as well. Keep in mind the teams are limited to 5 replacement parts for each key component in the engine. Rather than say 5 engines they can replace different pieces, but they can only replace 5 of them. After that the grid penalties get very confusing. We could be seeing penalties on a regular basis for the Renault based teams if they aren't able to resolve these issues in the next 4 weeks.
On the other hand Mercedes did 97 laps yesterday with Nico Rosberg and a total of 248 laps in the first two days. I believe last year day 1 at Jerez had ~800 laps of testing to give you an idea of just how difficult these first few days have been. So far Mercedes and Ferrari (works teams) appear to be the most reliable and after a hiccup on day 1 the McLaren looks to be in extremely good shape as well.