Sleepers usually don't have consistently terrible second halves like the infamous Jeremy Bonderman does.
I'll skip over his total numbers because that would just reinforce my point to where there should exist no counter argument, so let's look at his more recent numbers beginning from '06. Keep in mind, these are just the outside numbers that usually count in fantasy leagues, as I am skipping over his GB%, FB%, LD%, and other statistics.
'06 First Half Numbers:
119.2 IP, 111 Ks, 3.46 ERA. Fairly respectable numbers for a young SP.
94.1 IPs, 91 Ks, BUT a 4.87 ERA
'07 First Half:
106 IPs, 98 Ks, 3.48 ERA. About the same as his year before except he did add a great 9-1 record.
68.1 IPs, 47 Ks, and here's the kicker: 7.38 ERA.
It's been a consistent trend for Bonde to be absolutely awful in the second half, so here's my answer on if you should take a flier on Mr. Jeremy Bonderman: Unless you want a solid first-half SP and a completely different SP in the second half, go take a chance. Hey, maybe he'll come out of the disturbing trend, but if there needs to be more proof, I can always give a link to his fangraph.