It might counteract creatine in regards to intermittent maximal muscle contraction as assessed by leg extension, but the evidence is limited to one study. Additionally, when measuring performance (sprints, usually) where repeated short-term muscle contractions are the concern there is no difference between creatine or creatine+caffeine.

It does not seem to contradict any effects on anaerobic cardio (HIIT) and seems to actually be quite additive in its benefits on performance.

On muscle contraction

In active males, caffeine (5mg/kg bodyweight once daily at breakfast) coingestion with Creatine (0.5g/kg bodyweight over 8 doses) does not appear to matter in regards to muscular creatine content, but differences are noted on intermittent force production where creatine was significantly greater than placebo (0.5g/kg maltodextrin), but creatine + caffeine was not.[1]

'Intermittent Force Production', in accordance with the prior study,[1] is depicted below. Three overall 'bouts' of exercise descending from a triplet (3 sets) of 30 reps with 60 seconds rest, 4 sets of 20 reps with 40 seconds rest in between each one, and finally 5 sets of 10 reps with 20 seconds rest in between. 2 minutes rest between each of the three bouts. Each rep was to be a Maximal Voluntary Contraction (MVC)

B group is creatine supplementation, whereas C is creatine + caffeine.

On cardiovascular exercise

In high intensity exercise (125% VO2 max running) when subjects loaded for 6 days with 0.3g/kg bodyweight, a one-time acute load of 5mg/kg bodyweight caffeine does not appear to impair the effects of creatine supplementation on HIIT and exerts its own effects; the end result is that creatine + caffeine outperforms creatine alone and placebo.[2]

These results have been replicated with 10 second sprints on a cycling device, in which creatine was loaded for 6 days (at 0.3g/kg) and then acute ingestion of caffeine at 6mg/kg bodyweight outperformed the group that then received the acute placebo,[3] a pre-workout supplement consisting of creatine and caffeine (with amino acids) seems to be effective independent of loading when taken before high intensity exercise, but doesn't tend to increase longer aerobic exercise.[4][5][6]

Other notables

Effects on pharmacokinetics

Muscular loading does not seem to differ significantly between creatine and creatine+caffeine,[1] and the general serum pharmacokinetic profile does not seem to differ greatly either.[7]

Effects on water weight

There do not appear to be inconsistencies in water weight gain from creatine supplementation. Either all subjects gain weight[2] or none do.[1] No between group differences seem to exist.