Basic rule is: if you make the effort to arrive 10 minutes earlier than the bus that will get you in on time 50% of the time, you'll be fine. But what you will find is that everyone is lazy and they catch the bus later and later. Eventually, you're arriving at the bus stop at a time which would only get you to campus on time if the bus arrived as soon as you get to the bus stop. To be honest, there are 8 buses that arrive on campus at the 8am hour, plus 14 that will get you there between 9 and 10am. I've never really seen a problem with the 9am service because I think most who live in Leam actually start at 10, 11 or 12 and this is probably a busier time.
Pretty much the U1 will be the best service because:
1) It stops outside Gibbet Hill campus (X17 stops at the bottom of Gibbet Hill Road/Kenilworth Rd, OK so it's not much further)
2) When the U1 actually runs, it goes via the A46, rather than through Kenilworth.
3) Last bus home is later than the 12 on Mon-Fri and Saturday. 12 has a last bus leaving at around midnight on all nights.
4) U1s are more frequent than 12s (between campus and Leam)
Downsides are:
1) Services during public holidays and also on Sundays outside of term time (eg the Sunday before the start of term). The last bus is at like 7, whereas the 12s will still run until midnight.
2) U2/U17 which run on the weekends go through Kenilworth, and actually the route taken is longer than the 12s route through Kenilworth.
3) The U12 (Mon-Fri in the mornings, term time) runs the same route as the 12 (ie via Kenilworth). Which basically means any bus arriving on campus before 9 will go via Kenilworth anyway. I've successfully caught 12s from Leam (Parade) at around 8:20 and got to campus on time, so yeah aim for 8.10-8.20 and you will be fine.
http://www.stagecoachbus.com/PdfUploads/Timetable_22397_Unibus.pdfThat's the timetable.