I have the tool now and several O ring/battery kits. The tool is well made and it opens the Spyder very easily, I ordered four extra kits (the tool comes with one) and the total cost was around £40 for all inc. postage. So for less than the cost of a dealer battery change I can do five. I haven't yet dived with the computer but I don't envisage any flooding problems, I have changed many dive watch batteries and reused the O rings without ever having any floods. As long as you follow the comprehensive instructions all should be OK.
Definitely recommended
Steve