Its a great conversion. I had the 250 engine fitted nearly 2 years now. On the whole its been very reliable as I use it daily. Theres a couple of things I'd do differently if I was doing it again. I'd def buy the driveshaft's new, and the proper Renault bolts for the turbo manifold. I'd also go with SteC intercooler kit which wasn't available at the time.
The conversion can be as cheap or expensive like most things, depending on how you spec it. Gearbox wise, the clio box gearing makes it feel a little quicker but I think its the same in real terms. Cost also comes into it. To diff a clio box is £1000+, and you still have the issues relating to the box anyway especially when you think your putting double the torque through it. Meg R26 for me was a no brainer, comes with diff and good for 300 bhp+, and can be found for around £300. Also you have the options to upgrade the clutch which you don't with the Clio box in the same way. I use the car on the motorway, so the gearing in the Meg box suits me better too.
Like I said, its not plug n' play and you have to except that it takes time to work through some of the issues initially. If most people are honest, their cars (as mine has) have been back a few times to tweak stuff. It can be a lottery when buying a second hand engine and some people have been unlucky. Although some can do the conversion themselves, the diagnosis when it goes wrong (as all cars do at some point), or the car doesn't run, you cant just go to your regular local garage to sort out some of the issues, and you certainly cant go to Renault!!!!!
I would definitely recommend using someone who's done the conversion like SteC. Even the cars done by owners have still had the ecu and loom done by him I think. I know him personally but I can say its very different talking about the conversion than actually doing it, and without the ability to sort the immobiliser out, the conversion isn't happening.