My general advice would be to avoid Ryanair like the plague but it depends on the route. More relevant information would be helpful e.g.

1) Where and when are you flying to e.g. Aer Lingus or BMI are often cheaper when flying to Ireland. Also as stated above, the cost/time involved in getting from the arrival airport to your destination city often outweighs any savings made.

2) Are you planning a same day connection from your incoming US flight? If so, it's not worth the hassle or risk of connecting LHR/LGW to Stansted.

3) Do you like running? No pre-assigned seats means a sprint/scrum for the better seats onboard. Previous experience of rugby may also help.

All this is assuming things go to plan, and you don't have to rely on Ryanair's "customer service" dept.

On a positive note they have forced the major airlines to reduce their prices so I am grateful to those people who believe their "low price" marketing BS and continue to fly them.