Does caffeine improve performance for habituated consumers? I've seen a fair few studies over the while that suggest caffeine increases arousal in the short term and that for some tasks, this will also increase performance (there's a few citations here).

However, as with most drugs, habituation leads to withdrawal effects and adjustment, so that in some senses once a person gets used to caffeine, having a regular dose of caffeine may just bring a person to something like a normal level of functioning.

Thus, in a general sense, 
for someone used to consuming a certain level of caffeine, does having that regular quantity increase arousal or performance over and above the level that would be obtained at that moment had the person never got in the habit of consuming caffeine in the first place?
Nehligh's 2010 review

Nehlig (2010) wrote a systematic review article called "Is caffeine a cognitive enhancer?" (link to publisher, no PDF).
On page S89 to S90 Nehlig reviews the evidence of caffeine as a cognitive enhancer within the context of a history of caffeine consumption:


  In
  caffeine non-deprived young (20–25 years) and older subjects (50–65
  years), 250 mg caffeine slightly offset the decline in performance
  found in the placebo group on a digit span memory test. However,
  caffeine did not improve performance in the immediate word recall
  task in either age group [11]. Caffeine (12.5–100 mg) given after
  overnight abstinence improved cognitive performance in a relatively
  difficult and stressful task involving rapid visual information
  processing and using a high load of working memory in 11 males and 12
  females, aged 18–56 years. The effect was more marked in individuals
  with a high habitual caffeine intake [78]. In 68 volunteers
  consuming a regular daily amount of caffeine, 2 mg/kg caffeine
  improved the speed of encoding new information in a categorical search
  task [79]. Likewise, in a study comparing 24 non-consumers (20
  mg/day) to 24 consumers of caffeine (217 mg/day), there was no
  baseline difference between group performances. Caffeine improved
  numeric working memory reaction time and sentence verification
  accuracy; alertness was also increased, but in general caffeine tended
  to improve more the performance in non consumers [80]. 
  
  In adult
  subjects, a higher habitual caffeine intake with no additional
  caffeine administration on the day of testing was positively related
  to better performance on incidental verbal learning and visuospatial
  learning tasks [15]. There was also an association between estimated
  caffeine intake and performance in a choice reaction time task, and in
  delayed recall of a verbal word learning task [81]. In the former
  study, older sub- jects appeared to benefit more from higher caffeine
  intake [15], while this was not the case in the latter study [81].
  Also, in real life activities, regular caffeine consumption might
  benefit cognitive functioning in a non-working population [82].


Smith's 2002 review

Another good review is provided by Smith (2002, PDF).
The main claim relevant to the present question argues that coffee remains beneficial despite habituation:


  It has been claimed that the positive eﬀects of caﬀeine really reﬂect
  removal of negative eﬀects of caﬀeine withdrawal. This view cannot
  account for eﬀects observed in non-consumers or nondeprived
  individuals. In addition, there is little evidence of caﬀeine
  withdrawal impairing tasks which show improvements following ingestion
  of caﬀeine.


The article is worth reading if you want to read about the relevant empirical evidence and some of the limitations of existing research.

References:


[11] Rees K, Allen D, Lader M (1999) The influences of age and
caffeine on psychomotor and cognitive function. Psychophar-
macology (Berl) 145, 181-188.
[78] Smit HJ, Rogers PJ (2000) Effects of low doses of caffeine
on cognitive performance, mood and thirst in low and higher
caffeine consumers. Psychopharmacology (Berl) 152, 167-
173.
[80] Haskell CF, Kennedy DO, Wesnes KA, Scholey AB (2005)
Cognitive and mood improvements of caffeine in habitual con-
sumers and habitual non-consumers of caffeine. Psychophar-
macology (Berl) 179, 813-825.
[81] Hameleers PA, Van Boxtel MP, Hogervorst E, Riedel WJ,
Houx PJ, Buntinx F, Jolles J (2000) Habitual caffeine con-
sumption and its relation to memory, attention, planning
capacity and psychomotor performance across multiple age
groups. Hum Psychopharmacol 15, 573-581.
[82] Smith AP (2009) Caffeine, cognitive failures and health in a
non-working community sample. Hum Psychopharmacol 24,
29-34.
Smith, A. (2002). Effects of caffeine on human behavior. Food and chemical toxicology, 40, 1243-1255. PDF